The question is one of great complexity. Considered from the more
fundamental molecular point of view we should perhaps look to
failure of the power of cell division as the condition of
mortality. For it is to this phenomenon—that of cell
division—that the continued life of the protozoon is to be
ascribed, as we have already seen. Reproduction is, in fact, the
saving factor here.

As we do not know the source or nature of the stimulus

82

responsible for cell division we cannot give a molecular account
of death in the higher organisms. However we shall now see that,
philosophically, we are entitled to consider reproduction as a
saving factor in this case also; and to regard the death of the
individual much as we regard the fall of the leaf from the tree:
_i.e._ as the cessation of an outgrowth from a development
extending from the past into the future. The phenomena of old age
and natural death are, in short, not at variance with the
progressive activity of the organism. We perceive this when we
come to consider death from the evolutionary point of view.

Professor Weismann, in his two essays, "The Duration of Life,"
and "Life and Death,"[1] adopts and defends the view that "death
is not a primary necessity but that it has been secondarily
acquired by adaptation." The cell was not inherently limited in
its number of cell-generations. The low unicellular organisms are
potentially immortal, the higher multicellular forms with
well-differentiated organs contain the germs of death within
themselves.

He finds the necessity of death in its utility to the species.
Long life is a useless luxury. Early and abundant reproduction is
best for the species. An immortal individual would gradually
become injured and would be valueless or even harmful to the
species by taking the place of those that are sound. Hence
natural selection will shorten life.

[1] See his _Biological Memoirs._ Oxford, 1888.

83

Weismann contends against the transmission of acquired characters
as being unproved.[1] He bases the appearance of death on
variations in the reproductive cells, encouraged by the ceaseless
action of natural selection, which led to a differentiation into
perishable somatic cells and immortal reproductive cells. The
time-limit of any particular organism ultimately depends upon the
number of somatic cell-generations and the duration of each
generation. These quantities are "predestined in the germ itself"
which gives rise to each individual. "The existence of immortal
metazoan organisms is conceivable," but their capacity for
existence is influenced by conditions of the external world; this
renders necessary the process of adaptation. In fact, in the
differentiation of somatic from reproductive cells, material was
provided upon which natural selection could operate to shorten or
to lengthen the life of the individual in accordance with the
needs of the species. The soma is in a sense "a secondary
appendage of the real bearer of life—the reproductive cells." The
somatic cells probably lost their immortal qualities, on this
immortality becoming useless to the species. Their mortality may
have been a mere consequence of their differentiation (loc. cit.,
p. 140), itself due to natural selection. "Natural death was
not," in fact, "introduced from absolute intrinsic necessity
inherent in the nature of living matter, but on grounds of
utility,

[1] Biological Memoirs, p. 142.

84

that is from necessities which sprang up, not from the general
conditions of life, but from those special conditions which
dominate the life of multicellular organisms."

On the inherent immortality of life, Weismann finally states:
"Reproduction is, in truth, an essential attribute of living
matter, just as the growth which gives rise to it.... Life is
continuous, and not periodically interrupted: ever since its
first appearance upon the Earth in the lowest organism, it has
continued without break; the forms in which it is manifest have
alone undergone change. Every individual alive today—even the
highest—is to be derived in an unbroken line from the first and
lowest forms." [1]

At the present day the view is very prevalent that the soma of
higher organisms is, in a sense, but the carrier for a period of
the immortal reproductive cells (Ray Lankester)[2]—an appendage
due to adaptation, concerned in their supply, protection, and
transmission. And whether we regard the time-limit of its
functions as due to external constraints, recurrently acting till
their effects become hereditary, or to variations more directly
of internal origin, encouraged by natural selection, we see in
old age and death phenomena ultimately brought about in obedience
to the action of an environment. These are not inherent in the
properties of living matter. But, in spite

[1] Loc. cit., p. 159

[2] Geddes and Thomson, The Evolution of Sex, chap. xviii.

85

of its mortality, the body remains a striking manifestation of
the progressiveness of the organism, for to this it must be
ascribed. To it energy is available which is denied to the
protozoon. Ingenious adaptations to environment are more
especially its privilege. A higher manifestation, however, was
possible, and was found in the development of mind. This, too, is
a servant of the cell, as the genii of the lamp. Through it
energy is available which is denied to the body. This is the
masterpiece of the cell. Its activity dates, as it were, but from
yesterday, and today it inherits the most diverse energies of the
Earth.

Taking this view of organic succession, we may liken the
individual to a particle vibrating for a moment and then coming
to rest, but sweeping out in its motion one wave in the
continuous organic vibration travelling from the past into the
future. But as this vibration is one spreading with increased
energy from each vibrating particle, its propagation involves a
continual accelerated inflow of energy from the surrounding
medium, a dynamic condition unknown in periodic effects
transmitted by inanimate actions, and, indeed, marking the
fundamental difference between the dynamic attitudes of the
animate and inanimate.

We can trace the periodic succession of individuals on a diagram
of activity with some advantage. Considering, first, the case of
the unicellular organism reproducing by subdivision and recalling
that conditions, definite and inevitable, oppose a limit to the
rate of growth, or, for our

86

present purpose, rate of consumption of energy, we proceed as
follows:

{Fig. 1}

Along a horizontal axis units of time are measured; along a
vertical axis units of energy. Then the life-history of the
amoeba, for example, appears as a line such as A in Fig. 1.
During the earlier stages of its growth the rate of absorption of
energy is small; so that in the unit interval of time, t, the
small quantity of energy, e1, is absorbed. As life advances, the
activity of the organism augments, till finally this rate attains
a maximum, when e2 units of energy are consumed in the unit of
time.[1]

[1] Reference to p. 76, where the organic system is treated as
purely mechanical, may help readers to understand what is
involved in this curve. The solar engine may, unquestionably,
have its activity defined by such a curve. The organism is,
indeed, more complex; but neither this fact nor our ignorance of
its mechanism, affects the principles which justify the diagram.

87

On this diagram reproduction, on the part of the organism, is
represented by a line which repeats the curvature of the parent
organism originating at such a point as P in the path of the
latter, when the rate of consumption of energy has become
constant. The organism A has now ceased to act as a unit. The
products of fission each carry on the vital development of

{Fig. 2}

the species along the curve B, which may be numbered (2), to
signify that it represents the activity of two individuals, and
so on, the numbering advancing in geometrical progression. The
particular curvature adopted in the diagram is, of course,
imaginary; but it is not of an indeterminate nature. Its course
for any species is a characteristic of fundamental physical
importance, regarding the part played in nature by the particular
organism.

88

In Fig. 2 is represented the path of a primitive multicellular
organism before the effects of competition produced or fostered
its mortality. The lettering of Fig. 1 applies; the successive
reproductive acts are marked P1, P2; Q1, Q2, etc., in the paths
of the successive individuals.

{Fig. 3}

The next figure (Fig. 3) diagrammatically illustrates death in
organic history. The path ever turns more and more from the axis
of energy, till at length the point is reached when no more
energy is available; a tangent to the curve at this point is at
right angles to the axis of energy and parallel to the time axis.
The death point is reached, and however great a length we measure
along the axis of time, no further consumption of energy is

89

indicated by the path of the organism. Drawing the line beyond
the death point is meaningless for our present purpose.

It is observable that while the progress of animate nature finds
its representation on this diagram by lines sloping _upwards_ from
left to right, the course of events in inanimate nature—for
example, the history of the organic configuration after death, or

{Fig. 4}

the changes progressing—let us say, in the solar system, or in
the process of a crystallisation, would appear as lines sloping
downwards from left to right.

Whatever our views on the origin of death may be, we have to
recognise a periodicity of functions in the life-history of the
successive individuals of the present day; and whether or not we
trace this directly or indirectly to

90

a sort of interference with the rising wave of life, imposed by
the activity of a series of derived units, each seeking energy,
and in virtue of its adaptation each being more fitted to obtain
it than its predecessor, or even leave the idea of interference
out of account altogether in the origination or perpetuation of
death, the truth of the diagram (Fig. 4) holds in so far as it
may be supposed to graphically represent the dynamic history of
the individual. The point chosen on the curve for the origination
of a derived unit is only applicable to certain organisms, many
reproducing at the very close of life. A chain of units are
supposed here represented.[1]

THE LENGTH OF LIFE

If we lay out waves as above to a common scale of time for
different species, the difference of longevity is shown in the
greater or less number of vibrations executed in a given time,
_i.e._ in greater or less "frequency." We cannot indeed draw the
curvature correctly, for this would necessitate a knowledge which
we have not of the activity of the organism at different periods
of its life-history, and so neither can we plot the direction of
the organic line of propagation with respect to the

[1] Projecting upon the axes of time and energy any one complete
vibration, as in Fig. 4, the total energy consumed by the
organism during life is the length E on the axis of energy, and
its period of life is the length T on the time-axis. The mean
activity is the quotient E/T.

91

axes of reference as this involves a knowledge of the mean
activity.[1]

The group of curves which follow, relating to typical animals
possessing very different activities (Fig. 5), are therefore
entirely diagrammatic, except in respect to the approximate

{Fig. 5}

longevity of the organisms. (1) might represent an animal of the
length of life and of the activity of Man; (2), on the same scale
of longevity,

[1] In the relative food-supply at various periods of life the
curvature is approximately determinable.

92

one of the smaller mammals; and (3), the life-history of a cold
blooded animal living to a great age; _e.g._ certain of the
reptilia.

It is probable, that to conditions of structural development,
under the influence of natural selection, the question of longer
or shorter life is in a great degree referable. Thus, development
along lines of large growth will tend to a slow rate of
reproduction from the simple fact that unlimited energy to supply
abundant reproduction is not procurable, whatever we may assume
as to the strength or cunning exerted by the individual in its
efforts to obtain its supplies. On the other hand, development
along lines of small growth, in that reproduction is less costly,
will probably lead to increased rate of reproduction. It is, in
fact, matter of general observation that in the case of larger
animals the rate of reproduction is generally slower than in the
case of smaller animals. But the rate of reproduction might be
expected to have an important influence in determining the
particular periodicity of the organism. Were we to depict in the
last diagram, on the same time-scale as Man, the vibrations of
the smaller and shorter-lived living things, we would see but a
straight line, save for secular variations in activity,
representing the progress of the species in time: the tiny
thrills of its units lost in comparison with the yet brief period
of Man.

The interdependence of the rate of reproduction and

93

the duration of the individual is, indeed, very probably revealed
in the fact that short-lived animals most generally reproduce
themselves rapidly and in great abundance, and vice versa. In
many cases where this appears contradicted, it will be found that
the young are exposed to such dangers that but few survive (_e.g._
many of the reptilia, etc.), and so the rate of reproduction is
actually slow.

Death through the periodic rigour of the inanimate environment
calls forth phenomena very different from death introduced or
favoured by competition. A multiplicity of effects simulative of
death occur. Organisms will, for example, learn to meet very
rigorous conditions if slowly introduced, and not permanent. A
transitory period of want can be tided over by contrivance. The
lily withdrawing its vital forces into the bulb, protected from
the greatest extremity of rigour by seclusion in the Earth; the
trance of the hibernating animal; are instances of such
contrivances.

But there are organisms whose life-wave truly takes up the
periodicity of the Earth in its orbit. Thus the smaller animals
and plants, possessing less resources in themselves, die at the
approach of winter, propagating themselves by units which,
whether egg or seed, undergo a period of quiescence during the
season of want. In these quiescent units the energy of the
organism is potential, and the time-energy function is in
abeyance. This condition is, perhaps, foreshadowed in the
encyst-

94

ment of the amoeba in resistance to drought. In most cases of
hibernation the time-energy function seems maintained at a loss
of potential by the organism, a diminished vital consumption of
energy being carried on at the expense of the stored energy of
the tissues. So, too, even among the largest organisms there will
be a diminution of activity periodically inspired by
climatological conditions. Thus, wholly or in part, the activity
of organisms is recurrently affected by the great energy—tides
set up by the Earth's orbital motion.

{Fig. 6}

Similarly in the phenomenon of sleep the organism responds to the
Earth's axial periodicity, for in the interval of night a period
of impoverishment has to be endured. Thus the diurnal waves of
energy also meet a response in the organism. These tides and
waves of activity would appear as larger and smaller ripples

95

on the life-curve of the organism. But in some, in which life and
death are encompassed in a day, this would not be so; and for the
annual among plants, the seed rest divides the waves with lines
of no activity (Fig. 6).

Thus, finally, we regard the organism as a dynamic phenomenon
passing through periodic variations of intensity. The material
systems concerned in the transfer of the energy rise, flourish,
and fall in endless succession, like cities of ancient dynasties.
At points of similar phase upon the waves the rate of consumption
of energy is approximately the same; the functions, too, which
demand and expend the energy are of similar nature.

That the rhythm of these events is ultimately based on harmony in
the configuration and motion of the molecules within the germ
seems an unavoidable conclusion. In the life of the individual
rhythmic dynamic phenomena reappear which in some cases have no
longer a parallel in the external world, or under conditions when
the individual is no longer influenced by these external
conditions.,, In many cases the periodic phenomena ultimately die
out under new influences, like the oscillations of a body in a
viscous medium; in others when they seem to be more deeply rooted
in physiological conditions they persist.

The "length of life is dependent upon the number

[1] The _Descent of Man._

96

of generations of somatic cells which can succeed one another in
the course of a single life, and furthermore the number as well
as the duration of each single cell-generation is predestined in
the germ itself."[1]

Only in the vague conception of a harmonising or formative
structural influence derived from the germ, perishing in each
cell from internal causes, but handed from cell to cell till the
formative influence itself degrades into molecular discords, does
it seem possible to form any physical representation of the
successive events of life. The degradation of the molecular
formative influence might be supposed involved in its frequent
transference according to some such dynamic actions as occur in
inanimate nature. Thus, ultimately, to the waste within the cell,
to the presence of a force retardative of its perpetual harmonic
motions, the death of the individual is to be ascribed. Perhaps
in protoplasmic waste the existence of a universal death should
be recognised. It is here we seem to touch inanimate nature; and
we are led back to a former conclusion that the organism in its
unconstrained state is to be regarded as a contrivance for
evading the dynamic tendencies of actions in which lifeless
matter participates.[2]

[1] Weismann, _Life and Death; Biological Memoirs_, p. 146.

[2] In connection with the predestinating power and possible
complexity of the germ, it is instructive to reflect on the very
great molecular population of even the smallest spores—giving
rise to very simple forms. Thus, the spores of the unicellular
Schizomycetes are estimated to dimensions as low as 1/10,000 of a
millimetre in diameter (Cornil et Babes, _Les Batteries_, 1. 37).
From Lord Kelvin's estimate of the number of molecules in water,
comprised within the length of a wave-length of yellow light
(_The Size of Atoms_, Proc. R. I., vol. x., p. 185) it is
probable that such spores contain some 500,000 molecules, while
one hundred molecules range along a diameter.

97

THE NUMERICAL ABUNDANCE OF LIFE

We began by seeking in various manifestations of life a dynamic
principle sufficiently comprehensive to embrace its very various
phenomena. This, to all appearance, found, we have been led to
regard life, to a great extent, as a periodic dynamic phenomenon.
Fundamentally, in that characteristic of the contrivance, which
leads it to respond favourably to transfer of energy, its
enormous extension is due. It is probable that to its instability
its numerical abundance is to be traced—for this, necessitating
the continual supply of all the parts already formed, renders
large, undifferentiated growth, incompatible with the limited
supplies of the environment. These are fundamental conditions of
abundant life upon the Earth.

Although we recognise in the instability of living systems the
underlying reason for their numerical abundance, secondary
evolutionary causes are at work. The most important of these is
the self-favouring nature of the phenomenon of reproduction. Thus
there is a tendency not only to favour reproductiveness, but
early reproductiveness, in the form of one prolific
reproductive.

98

act, after which the individual dies.[1] Hence the wavelength of
the species diminishes, reproduction is more frequent, and
correspondingly greater numbers come and go in an interval of
time.

Another cause of the numerical abundance of life exists, as
already stated, in the conditions of nourishment. Energy is more
readily conveyed to the various parts of the smaller mass, and
hence the lesser organisms will more actively functionate; and
this, as being the urging dynamic attitude, as well as that most
generally favourable in the struggle, will multiply and favour
such forms of life. On the other hand, however, these forms will
have less resource within themselves, and less power of
endurance, so that they are only suitable to fairly uniform
conditions of supply; they cannot survive the long continued want
of winter, and so we have the seasonal abundance of summer. Only
the larger and more resistant organisms, whether animal or
vegetable, will, in general, populate the Earth from year to
year. From this we may conclude that, but for the seasonal
energy-tides, the development of life upon the globe had gone
along very different lines from those actually followed. It is,
indeed, possible that the evolution of the larger organisms would
not have occurred; there would have been no vacant place for
their development, and a being so endowed as Man could hardly

[1] Weismann, _The Duration of Life._

99

have been evolved. We may, too, apply this reasoning elsewhere,
and regard as highly probable, that in worlds which are without
seasonal influences, the higher developments of life have not
appeared; except they have been evolved under other conditions,
when they might for a period persist. We have, indeed, only to
picture to ourselves what the consequence of a continuance of
summer would be on insect life to arrive at an idea of the
antagonistic influences obtaining in such worlds to the survival
of larger organisms.

It appears that to the dynamic attitude of life in the first
place, and secondarily to the environmental conditions limiting
undifferentiated growth, as well as to the action of heredity in
transmitting the reproductive qualities of the parent to the
offspring, the multitudes of the pines, and the hosts of ants,
are to be ascribed. Other causes are very certainly at work, but
these, I think, must remain primary causes.

We well know that the abundance of the ants and pines is not a
tithe of the abundance around us visible and invisible. It is a
vain endeavour to realise the countless numbers of our
fellow-citizens upon the Earth; but, for our purpose, the
restless ants, and the pines solemnly quiet in the sunshine, have
served as types of animate things. In the pine the gates of the
organic have been thrown open that the vivifying river of energy
may flow in. The ants and the butterflies sip for a brief moment
of its waters, and again vanish into the

100

inorganic: life, love and death encompassed in a day.

Whether the organism stands at rest and life comes to it on the
material currents of the winds and waters, or in the vibratory
energy of the æther; or, again, whether with restless craving it
hurries hither and thither in search of it, matters nothing. The
one principle—the accelerative law which is the law of the
organic—urges all alike onward to development, reproduction and
death. But although the individual dies death is not the end; for
life is a rhythmic phenomenon. Through the passing ages the waves
of life persist: waves which change in their form and in the
frequency to which they are attuned from one geologic period to
the next, but which still ever persist and still ever increase.
And in the end the organism outlasts the generations of the
hills.

101

THE BRIGHT COLOURS OF ALPINE FLOWERS [1]

IT is admitted by all observers that many species of flowering
plants growing on the higher alps of mountainous regions display
a more vivid and richer colour in their bloom than is displayed
in the same species growing in the valleys. That this is actually
the case, and not merely an effect produced upon the observer by
the scant foliage rendering the bloom more conspicuous, has been
shown by comparative microscopic examination of the petals of
species growing on the heights and in the valleys. Such
examination has revealed that in many cases pigment granules are
more numerous in the individuals growing at the higher altitudes.
The difference is specially marked in Myosotis sylvatica,
Campanula rotundifolia, Ranunculus sylvaticus, Galium cruciatum,
and others. It is less marked in the case of Thymus serpyllum and
Geranium sylvaticum; while in Rosa alpina and Erigeron alpinus no
difference is observable.[2]

In the following cases a difference of intensity of colour is,
according to Kerner ("Pflanzenleben," 11. 504), especially
noticeable:— _Agrostemma githago, Campanula

[1] _Proc. Royal Dublin Society_, 1893.

[2] G. Bonnier, quoted by De Varigny, _Experimental Evolution_,
p. 55.

102

pusilla, Dianthus inodorus (silvestris), Gypsophila repens, Lotus
corniculatus, Saponaria ocymoides, Satureja hortensis, Taraxacumm
officinale, Vicia cracca, and Vicia sepium._

To my own observation this beautiful phenomenon has always
appeared most obvious and impressive. It appears to have struck
many unprofessional observers. Helmholtz offers the explanation
that the vivid colours are the result of the brighter sunlight of
the heights. It has been said, too, that they are the direct
chemical effects of a more highly ozonized atmosphere. The latter
explanation I am unable to refer to its author. The following
pages contain a suggestion on the matter, which occurred to me
while touring, along with Henry H. Dixon, in the Linthal district
of Switzerland last summer.[1]

If the bloom of these higher alpine flowers is especially
pleasing to our own æsthetic instincts, and markedly conspicuous
to us as observers, why not also especially attractive and
conspicuous to the insect whose mission it is to wander from
flower to flower over the pastures? The answer to this question
involves the hypothesis I would advance as accounting for the
bright colours of high-growing individuals. In short, I believe a
satisfactory explanation is to be found in the conditions of
insect life in the higher alps.

In the higher pastures the summer begins late and

[1] The summer of 1892.

103

closes early, and even in the middle of summer the day closes in
with extreme cold, and the cold of night is only dispelled when
the sun is well up. Again, clouds cover the heights when all is
clear below, and cold winds sweep over them when there is warmth
and shelter in the valleys. With these rigorous conditions the
pollinating insects have to contend in their search for food, and
that when the rival attractions of the valleys below are so many.
I believe it is these rigorous conditions which are indirectly
responsible for the bright colours of alpine flowers. For such
conditions will bring about a comparative scarcity of insect
activity on the heights; and a scarcity or uncertainty in the
action of insect agency in effecting fertilization will intensify
the competition to attract attention, and only the brightest
blooms will be fertilized.[1]

This will be a natural selection of the brightest, or the

[1] Grant Allen, I have recently learned, advances in _Science in
Arcady_ the theory that there is a natural selective cause
fostering the bright blooms of alpines. The selective cause is,
however, by him referred to the greater abundance of butterfly
relatively to bee fertilizers. The former, he says, display more
æsthetic instinct than bees. In the valley the bees secure the
fertilization of all. I may observe that upon the Fridolins Alp
all the fertilizers we observed were bees. I have always found
butterflies very scarce at altitudes of 7,000 to 8,000 feet. The
alpine bees are very light in body, like our hive bee, and I do
not think rarefaction of the atmosphere can operate to hinder its
ascent to the heights, as Grant Allen suggests. The observations
on the death-rate of bees and butterflies on the glacier, to be
referred to presently, seem to negative such a hypothesis, and to
show that a large preponderance of bees over butterflies make
their way to the heights.

104

brightest will be the fittest, and this condition, along with the
influence of heredity, will encourage a race of vivid flowers. On
the other hand, the more scant and uncertain root supply, and the
severe atmospheric conditions, will not encourage the grosser
struggle for existence which in the valleys is carried on so
eagerly between leaves and branches—the normal offensive and
defensive weapons of the plant—and so the struggle becomes
refined into the more æsthetic one of colour and brightness
between flower and flower. Hence the scant foliage and vivid
bloom would be at once the result of a necessary economy, and a
resort to the best method of securing reproduction under the
circumstances of insect fertilizing agency. Or, in other words,
while the luxuriant growth is forbidden by the conditions, and
thus methods of offence and defence, based upon vigorous
development, reduced in importance, it would appear that the
struggle is mainly referred to rivalry for insect preference. It
is probable that this is the more economical manner of carrying
on the contest.

In the valleys we see on every side the struggle between the
vegetative organs of the plant; the soundless battle among the
leaves and branches. The blossom here is carried aloft on a
slender stem, or else, taking but a secondary part in the
contest, it is relegated to obscurity (P1. XII.). Further up on
the mountains, where the conditions are more severe and the
supplies less abundant, the leaf and branch assume lesser
dimensions, for they are costly weapons to provide and the
elements are unfriendly

105

to their existence (Pl. XIII.). Still higher, approaching the
climatic limit of vegetable life, the struggle for existence is
mainly carried on by the æsthetic rivalry of lowly but
conspicuous blossoms.

As regards the conditions of insect life in the higher alps, it
came to my notice in a very striking manner that vast numbers of
such bees and butterflies as venture up perish in the cold of
night time. It appears as if at the approach of dusk these are
attracted by the gleam of the snow, and quitting the pastures,
lose themselves upon the glaciers and firns, there to die in
hundreds. Thus in an ascent of the Tödi from the Fridolinshüte we
counted in the early dawn sixty-seven frozen bees, twenty-nine
dead butterflies, and some half-dozen moths on the Biferten
Glacier and Firn. These numbers, it is to be remembered, only
included those lying to either side of our way over the snow, so
that the number must have mounted up to thousands when integrated
over the entire glacier and firn. Approaching the summit none
were found. The bees resembled our hive bee in appearance, the
butterflies resembled the small white variety common in our
gardens, which has yellow and black upon its wings. One large
moth, striped across the abdomen, and measuring nearly two inches
in length of body, was found. Upon our return, long after the
sun's rays had grown strong, we observed some of the butterflies
showed signs of reanimation. We descended so quickly to avoid the
inconvenience of the soft snow that we had time for no

106

close observation on the frozen bees. But dead bees are common
objects upon the snows of the alps.

These remarks I noted down roughly while at Linthal last summer,
but quite recently I read in Natural Science[1] the following
note:

"Late Flowering Plants.—While we write, the ivy is in flower, and
bees, wasps, and flies are jostling each other and struggling to
find standing-room on the sweet-smelling plant. How great must be
the advantage obtained by this plant through its exceptional
habit of flowering in the late autumn, and ripening its fruit in
the spring. To anyone who has watched the struggle to approach
the ivy-blossom at a time when nearly all other plants are bare,
it is evident that, as far as transport of pollen and
cross-fertilization go, the plant could not flower at a more
suitable time. The season is so late that most other plants are
out of flower, but yet it is not too late for many insects to be
brought out by each sunny day, and each insect, judging by its
behaviour, must be exceptionally hungry.

"Not only has the ivy the world to itself during its flowering
season, but it delays to ripen its seed till the spring, a time
when most other plants have shed their seed, and most edible
fruits have been picked by the birds. Thus birds wanting fruit in
the spring can obtain little but ivy, and how they appreciate the
ivy berry is evident

[1] For December, 1892, vol. i., p. 730.

107

by the purple stains everywhere visible within a short distance
of the bush."

These remarks suggest that the ivy adopts the converse attitude
towards its visitors to that forced upon the alpine flower. The
ivy bloom is small and inconspicuous, but then it has the season
to itself, and its inconspicuousness is no disadvantage, _i.e._
if one plant was more conspicuous than its neighbours, it would
not have any decided advantage where the pollinating insect is
abundant and otherwise unprovided for. Its dark-green berries in
spring, which I would describe as very inconspicuous, have a
similar advantage in relation to the necessities of bird life.

The experiments of M. C. Flahault must be noticed. This
naturalist grew seeds of coloured flowers which had ripened in
Paris, part in Upsala, and part in Paris; and seed which had
ripened in Upsala, part at Paris, and part at Upsala. The flowers
opening in the more northern city were in most cases the
brighter.[1] If this observation may be considered indisputable,
as appears to be the case, the question arises, Are we to regard
this as a direct effect of the more rigorous climate upon the
development of colouring matter on the blooms opening at Upsala?
If we suppose an affirmative answer, the theory of direct effect
by sun brightness must I think be abandoned. But I venture to
think that the explanation of the Upsala

[1] Quoted by De Varigny, _Experimental Evolution_, p. 56.

108

experiment is not to be found in direct climatic influence upon
the colour, but in causes which lie deeper, and involve some
factors deducible from biological theory.

The organism, as a result of the great facts of heredity and of
the survival of the fittest, is necessarily a system which
gathers experience with successive generations; and the principal
lesson ever being impressed upon it by external events is
economy. Its success depends upon the use it makes of its
opportunities for the reception of energy and the economy
attained in disposing of what is gained.

With regard to using the passing opportunity the entire seasonal
development of life is a manifestation of this attitude, and the
fleetness, agility, etc., of higher organisms are developments in
this direction. The higher vegetable organism is not locomotory,
save in the transferences of pollen and seed, for its food comes
to it, and the necessary relative motion between food and
organism is preserved in the quick motion of radiated energy from
the sun and the slower motion of the winds on the surface of the
earth. But, even so, the vegetable organism must stand ever ready
and waiting for its supplies. Its molecular parts must be ready
to seize the prey offered to it, somewhat as the waiting spider
the fly. Hence, the plant stands ready; and every cloud with
moving shadow crossing the fields handicaps the shaded to the
benefit of the unshaded plant in the adjoining field. The open
bloom

109

is a manifestation of the generally expectant attitude of the
plant, but in relation to reproduction.

As regards economy, any principle of maximum economy, where many
functions have to be fulfilled, will, we may very safely predict,
involve as far as possible mutual helpfulness in the processes
going on. Thus the process of the development towards meeting any
particular external conditions, A, suppose, will, if possible,
tend to forward the development towards meeting conditions B; so
that, in short, where circumstances of morphology and physiology
are favourable, the ideally economical system will be attained
when in place of two separate processes, a, ß, the one process y,
cheaper than a + ß, suffices to advance development
simultaneously in both the directions A and B. The economy is as
obvious as that involved in "killing two birds with the one
stone"—if so crude a simile is permissible—and it is to be
expected that to foster such economy will be the tendency of
evolution in all organic systems subjected to restraints as those
we are acquainted with invariably are.

Such economy might be simply illustrated by considering the case
of a reservoir of water elevated above two hydraulic motors, so
that the elevated mass of water possessed gravitational
potential. The available energy here represents the stored-up
energy in the organism. How best may the water be conveyed to the
two motors [the organic systems reacting towards conditions A and
B] so

110

that as little energy as possible is lost in transit? If the
motors are near together it is most economical to use the one
conduit, which will distribute the requisite supply of water to
both. If the motors are located far asunder it will be most
economical to lay separate conduits. There is greatest economy in
meeting a plurality of functions by the same train of
physiological processes where this is consistent with meeting
other demands necessitated by external or internal conditions.

But an important and obvious consequence arises in the supply of
the two motors from the one conduit. We cannot work one motor
without working the other. If we open a valve in the conduit both
motors start into motion and begin consuming the energy stored in
the tank. And although they may both under one set of conditions
be doing useful and necessary work, in some other set of
conditions it may be needless for both to be driven.

This last fact is an illustration of a consideration which must
enter into the phenomenon which an eminent biologist speaks of as
physiological or unconscious "memory,"[1] For the development of
the organism from the ovum is but the starting of a train of
interdependent events of a complexity depending upon the
experience of the past.

[1] Ewald Hering, quoted by Ray Lankaster, _The Advancement of
Science_, p. 283.

111

In short, we may suppose the entire development of the plant,
towards meeting certain groups of external conditions,
physiologically knit together according as Nature tends to
associate certain groups of conditions. Thus, in the case in
point, climatic rigour and scarcity of pollinating agency will
ever be associated; and in the long experience of the past the
most economical physiological attitude towards both is, we may
suppose, adopted; so that the presence of one condition excites
the apparent unconscious memory of the other. In reality the
process of meeting the one condition involves the process and
development for meeting the other.

And this consideration may be extended very generally to such
organisms as can survive under the same associated natural
conditions, for the history of evolution is so long, and the
power of locomotion so essential to the organism at some period
in its life history, that we cannot philosophically assume a
local history for members of a species even if widely severed
geographically at the present day. At some period in the past
then, it is very possible that the individuals today thriving at
Paris, acquired the experience called out at Upsala. The
perfection of physiological memory inspires no limit to the date
at which this may have occurred—possibly the result of a
succession of severe seasons at Paris; possibly the result of
migrations —and the seed of many flowering plants possess means
of migration only inferior to those possessed by the flying and
swimming animals. But, again, possibly the experi-

112

ence was acquired far back in the evolutionary history of the
flower.[1]

But a further consideration arises. Not only at each moment in
the life of the individual must maximum income and most judicious
expenditure be considered, but in its whole life history, and
even over the history of its race, the efficiency must tend to be
a maximum. This principle is even carried so far that when
necessary it leads to the death of the individual, as in the case
of those organisms which, having accomplished the reproductive
act, almost immediately expire. This view of nature may be
repellent, but it is, nevertheless, evident that we are parts of
a system which ruthlessly sacrifices the individual on general
grounds of economy. Thus, if the curve which defines the mean
rate of reception of energy of all kinds at different periods in
the life of the organism be opposed by a second curve, drawn
below the axis along which time is measured, representing the
mean rate of expenditure of energy on development, reproduction,
etc. (Fig. 7), this latter curve, which is, of course,

[1] The blooms of self-fertilising, and especially of
cleistogamic plants (_e.g._ Viola), are examples of unconscious
memory, or unconscious "association of ideas" leading to the
development of organs now functionless. The _Pontederia crassipes_
of the Amazon, which develops its floating bladders when grown in
water, but aborts them rapidly when grown on land, and seems to
retain this power of adaptation to the environment for an
indefinite period of time, must act in each case upon an
unconscious memory based upon past experience. Many other cases
might be cited.

113

physiologically dependent on the former, must be of such a nature
from its origin to its completion in death, that the condition is
realized of the most economical rate of expenditure at each
period of life.[1] The rate of expenditure of energy at any
period of life is, of course, in such a curve defined by the
slope of the curve towards the axis of time at the period in
question; but this particular slope _must be led to by a previous
part of the curve, and involves its past and future course to a
very great extent_.

{Fig. 7}

There will, therefore, be impressed upon the
organism by the factors of evolution a unified course of
economical expenditure completed only by its death, and which
will give to the developmental progress of the individual its
prophetic character.

In this way we look to the unified career of each organic unit,
from its commencement in the ovum to the day

[1] See _The Abundance of Life_.

114

when it is done with vitality, for that preparation for momentous
organic events which is in progress throughout the entire course
of development; and to the economy involved in the welding of
physiological processes for the phenomenon of physiological
memory, wherein we see reflected, as it were, in the development
of the organism, the association of inorganic restraints
occurring in nature which at some previous period impressed
itself upon the plastic organism. We may picture the seedling at
Upsala, swayed by organic memory and the inherited tendency to an
economical preparation for future events, gradually developing
towards the æsthetic climax of its career. In some such manner
only does it appear possible to account for the prophetic
development of organisms, not alone to be observed in the alpine
flowers, but throughout nature.

And thus, finally, to the effects of natural selection and to
actions defined by general principles involved in biology, I
would refer for explanation of the manner in which flowers on the
Alps develop their peculiar beauty.

115

MOUNTAIN GENESIS

OUR ancestors regarded mountainous regions with feelings of
horror, mingled with commiseration for those whom an unkindly
destiny had condemned to dwell therein. We, on the other hand,
find in the contemplation of the great alps of the Earth such
peaceful and elevated thoughts, and such rest to our souls, that
it is to those very solitudes we turn to heal the wounds of ife.
It is difficult to explain the cause of this very different point
of view. It is probably, in part, to be referred to that cloud of
superstitious horror which, throughout the Middle Ages, peopled
the solitudes with unknown terrors; and, in part, to the
asceticism which led the pious to regard the beauty and joy of
life as snares to the soul's well-being. In those eternal
solitudes where the overwhelming forces of Nature are most in
evidence, an evil principle must dwell or a dragon's dreadful
brood must find a home.

But while in our time the aesthetic aspect of the hills appeals
to all, there remains in the physical history of the mountains
much that is lost to those who have not shared in the scientific
studies of alpine structure and genesis. They lose a past history
which for interest com-

116

petes with anything science has to tell of the changes of the
Earth.

Great as are the physical features of the mountains compared with
the works of Man, and great as are the forces involved compared
with those we can originate or control, the loftiest ranges are
small contrasted with the dimensions of the Earth. It is well to
bear this in mind. I give here (Pl. XV.) a measured drawing
showing a sector cut from a sphere of 50 cms. radius; so much of
it as to exhibit the convergence of its radial boundaries which
if prolonged will meet at the centre. On the same scale as the
radius the diagram shows the highest mountains and the deepest
ocean. The average height of the land and the average depth of
the ocean are also exhibited. We see how small a movement of the
crust the loftiest elevation of the Himalaya represents and what
a little depression holds the ocean.

Nevertheless, it is not by any means easy to explain the genesis
of those small elevations and depressions. It would lead us far
from our immediate subject to discuss the various theoretical
views which have been advanced to account for the facts. The idea
that mountain folds, and the lesser rugosities of the Earth's
surface, arose in a wrinkling of the crust under the influence of
cooling and skrinkage of the subcrustal materials, is held by
many eminent geologists, but not without dissent from others.

The most striking observational fact connected with mountain
structure is that, without exception, the ranges

117

of the Earth are built essentially of sedimentary rocks: that is
of rocks which have been accumulated at some remote past time
beneath the surface of the ocean. A volcanic core there may
sometimes be—probably an attendant or consequence of the
uplifting—or a core of plutonic igneous rocks which has arisen
under the same compressive forces which have bowed and arched the
strata from their original horizontal position. It is not
uncommon to meet among unobservant people those who regard all
mountain ranges as volcanic in origin. Volcanoes, however, do not
build mountain ranges. They break out as more or less isolated
cones or hills. Compare the map of the Auvergne with that of
Switzerland; the volcanoes of South Italy with the Apennines.
Such great ranges as those which border with triple walls the
west coast of North America are in no sense volcanic: nor are the
Pyrenees, the Caucasus, or the Himalaya. Volcanic materials are
poured out from the summits of the Andes, but the range itself is
built up of folded sediments on the same architecture as the
other great ranges of the Earth.

Before attempting an explanation of the origin of the mountains
we must first become more closely acquainted with the phenomena
attending mountain elevation.

At the present day great accumulations of sediment are taking
place along the margins of the continents where the rivers reach
the ocean. Thus, the Gulf of Mexico receiving the sediment of the
Mississippi and Rio Grande;

118

the northeast coast of South America receiving the sediments of
the Amazons; the east coast of Asia receiving the detritus of the
Chinese rivers; are instances of such areas of deposition. Year
by year, century by century, the accumulation progresses, and as
it grows the floor of the sea sinks under the load. Of the
yielding of the crust under the burthen of the sediments we are
assured; for otherwise the many miles of vertically piled strata
which are uplifted to our view in the mountains, never could have
been deposited in the coastal seas of the past. The flexure and
sinking of the crust are undeniable realities.

Such vast subsiding areas are known as geosynclines. From the
accumulated sediments of the geosynclines the mountain ranges of
the past have in every case originated; and the mountains of the
future will assuredly arise and lofty ranges will stand where now
the ocean waters close over the collecting sediments. Every
mountain range upon the Earth enforces the certainty of this
prediction.

The mountain-forming movement takes place after a certain great
depth of sediment is collected. It is most intense where the
thickness of deposit is greatest. We see this when we examine the
structure of our existing mountain ranges. At either side where
the sediments thin out, the disturbance dies away, till we find
the comparatively shallow and undisturbed level sediments which
clothe the continental surface.

Whatever be the connection between the deposition and

119

the subsequent upheaval, _the element of great depth of
accumulation seems a necessary condition and must evidently enter
as a factor into the Physical Processes involved_. The mountain
range can only arise where the geosyncline is deeply filled by
long ages of sedimentation.

Dana's description of the events attending mountain building is
impressive:

"A mountain range of the common type, like that to which the
Appalachians belong, is made out of the sedimentary formations of
a long preceding era; beds that were laid down conformably, and
in succession, until they had reached the needed thickness; beds
spreading over a region tens of thousands of square miles in
area. The region over which sedimentary formations were in
progress in order to make, finally, the Appalachian range,
reached from New York to Alabama, and had a breadth of 100 to 200
miles, and the pile of horizontal beds along the middle was
40,000 feet in depth. The pile for the Wahsatch Mountains was
60,000 feet thick, according to King. The beds for the
Appalachians were not laid down in a deep ocean, but in shallow
waters, where a gradual subsidence was in progress; and they at
last, when ready for the genesis, lay in a trough 40,000 feet
deep, filling the trough to the brim. It thus appears that epochs
of mountain-making have occurred only after long intervals of
quiet in the history of a continent."[1]

[1] Dana, _Manual of Geology_, third edition, p. 794

120

On the western side of North America the work of
mountain-building was, indeed, on the grandest scale. For long
ages and through a succession of geological epochs, sedimentation
had proceeded so that the accumulations of Palaeozoic and
Mesozoic times had collected in the geosyncline formed by their
own ever increasing weight. The site of the future Laramide range
was in late Cretaceous times occupied by some 50,000 feet of
sedimentary deposits; but the limit had apparently been attained,
and at this time the Laramide range, as well as its southerly
continuation into the United States, the Rockies, had their
beginning. Chamberlin and Salisbury[1] estimate that the height
of the mountains developed in the Laramide range at this time was
20,000 feet, and that, owing to the further elevation which has
since taken place, from 32,000 to 35,000 feet would be their
present height if erosion had not reduced them. Thus on either
side of the American continent we have the same forces at work,
throwing up mountain ridges where the sediments had formerly been
shed into the ocean.

These great events are of a rhythmic character; the crust, as it
were, pulsating under the combined influences of sedimentation
and denudation. The first involves downward movements under a
gathering load, and ultimately a reversal of the movement to one
of upheaval; the second factor, which throughout has been in

[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.

121

operation as creator of the sediments, then intervenes as an
assailant of the newly-raised mountains, transporting their
materials again to the ocean, when the rhythmic action is
restored to its first phase, and the age-long sequence of events
must begin all over again.

It has long been inferred that compressive stress in the crust
must be a primary condition of these movements. The wvork
required to effect the upheavals must be derived from some
preexisting source of energy. The phenomenon—intrinsically one of
folding of the crust—suggests the adjustment of the earth-crust
to a lessening radius; the fact that great mountain-building
movements have simultaneously affected the entire earth is
certainly in favour of the view that a generally prevailing cause
is at the basis of the phenomenon.

The compressive stresses must be confined to the upper few miles
of the crust, for, in fact, the downward increase of temperature
and pressure soon confers fluid properties on the medium, and
slow tangential compression results in hydrostatic pressure
rather than directed stresses. Thus the folding visible in the
mountain range, and the lateral compression arising therefrom,
are effects confined to the upper parts of the crust.

The energy which uplifts the mountain is probably a surviving
part of the original gravitational potential energy of the crust
itself. It must be assumed that the crust in following downwards
the shrinking subcrustal magma, develops immense compressive
stresses in

122

its materials, vast geographical areas being involved. When
folding at length takes place along the axis of the elongated
syncline of deposition, the stresses find relief probably for
some hundreds of miles, and the region of folding now becomes
compressed in a transverse direction. As an illustration, the
Laramide range, according to Dawson, represents the reduction of
a surface-belt 50 miles wide to one of 25 miles. The marvellous
translatory movements of crustal folds from south to north
arising in the genesis of the Swiss Alps, which recent research
has brought to light, is another example of these movements of
relief, which continue to take place perhaps for many millions of
years after they are initiated.

The result of this yielding of the crust is a buckling of the
surface which on the whole is directed upwards; but depression
also is an attendant, in many cases at least, on mountain
upheaval. Thus we find that the ocean floor is depressed into a
syncline along the western coast of South America; a trough
always parallel to the ranges of the Andes. The downward
deflection of the crust is of course an outcome of the same
compressive stresses which elevate the mountain.

The fact that the yielding of the crust is always situated where
the sediments have accumulated to the greatest depth, has led to
attempts from time to time of establishing a physical connexion
between the one and the other. The best-known of these theories
is that of Babbage and Herschel. This seeks the connexion in the
rise of the

123

geotherms into the sinking mass of sediment and the consequent
increase of temperature of the earth-crust beneath. It will be
understood that as these isogeotherms, or levels at which the
temperature is the same, lie at a uniform distance from the
surface all over the Earth, unless where special variations of
conductivity may disturb them, the introduction of material
pressed downwards from above must result in these materials
partaking of the temperature proper to the depth to which they
are depressed. In other words the geotherms rise into the sinking
sediments, always, however, preserving their former average
distance from the surface. The argument is that as this process
undoubtedly involves the heating up of that portion of the crust
which the sediments have displaced downwards, the result must be
a local enfeeblement of the crust, and hence these areas become
those of least resistance to the stresses in the crust.

When this theory is examined closely, we see that it only amounts
to saying that the bedded rocks, which have taken the place of
the igneous materials beneath, as a part of the rigid crust of
the Earth, must be less able to withstand compressive stress than
the average crust. For there has been no absolute rise of the
geotherms, the thermal conductivities of both classes of
materials differing but little. Sedimentary rock has merely taken
the place of average crust-rock, and is subjected to the same
average temperature and pressure prevailing in the surrounding
crust. But are there any grounds for the

124

assumption that the compressive resistance of a complex of
sedimentary rocks is inferior to one of igneous materials? The
metamorphosed siliceous sediments are among the strongest rocks
known as regards resistance to compressive stress; and if
limestones have indeed plastic qualities, it must be remembered
that their average amount is only some 5 per cent. of the whole.
Again, so far as rise of temperature in the upper crust may
affect the question, a temperature which will soften an average
igneous rock will not soften a sedimentary rock, for the reason
that the effect of solvent denudation has been to remove those
alkaline silicates which confer fusibility.

When, however, we take into account the radioactive content of
the sediments the matter assumes a different aspect.

The facts as to the general distribution of radioactive
substances at the surface, and in rocks which have come from
considerable depths in the crust, lead us to regard as certain
the widespread existence of heat-producing radioactive elements
in the exterior crust of the Earth. We find, indeed, in this fact
an explanation—at least in part—of the outflow of heat
continually taking place at the surface as revealed by the rising
temperature inwards. And we conclude that there must be a
thickness of crust amounting to some miles, containing the
radioactive elements.

Some of the most recent measurements of the quantities of radium
and thorium in the rocks of igneous origin—_e.g._ granites,
syenites, diorites, basalts, etc., show that the

125

radioactive heat continually given out by such rocks amounts to
about one millionth part of 0.6 calories per second per cubic
metre of average igneous rock. As we have to account for the
escape of about 0.0014 calorie[1] per square metre of the Earth's
surface per second (assuming the rise of temperature downwards,
_i.e._ the "gradient" of temperature, to be one degree centigrade
in 35 metres) the downward extension of such rocks might, _prima
facie_, be as much as 19 kilometres.

About this calculation we have to observe that we assume the
average radioactivity of the materials with which we have dealt
at the surface to extend uniformly all the way down, _i.e._ that
our experiments reveal the average radioactivity of a radioactive
crust. There is much to be said for this assumption. The rocks
which enter into the measurements come from all depths of the
crust. It is highly probable that the less silicious, _i.e._ the
more basic, rocks, mainly come from considerable depths; the more
acid or silica-rich rocks, from higher levels in the crust. The
radioactivity determined as the mean of the values for these two
classes of rock closely agrees with that found for intermediate
rocks, or rocks containing an intermediate amount of silica.
Clarke contends that this last class of material probably
represents the average composition of the Earth's crust so far as
it has been explored by us.

[1] The calorie referred to is the quantity of heat required to
heat one gram of water, _i.e._ one cubic centimetre of
water—through one degree centigrade.

126

It is therefore highly probable that the value found for the mean
radioactivity of acid and basic rocks, or that found for
intermediate rocks, truly represents the radioactive state of the
crust to a considerable depth. But it is easy to show that we
cannot with confidence speak of the thickness of this crust as
determinable by equating the heat outflow at the surface with the
heat production of this average rock.

This appears in the failure of a radioactive layer, taken at a
thickness of about 19-kilometres, to account for the deep-seated
high temperatures which we find to be indicated by volcanic
phenomena at many places on the surface. It is not hard to show
that the 19-kilometre layer would account for a temperature no
higher than about 270° >C. at its base.

It is true that this will be augmented beneath the sedimentary
deposits as we shall presently see; and that it is just in
association with these deposits that deep-seated temperatures are
most in evidence at the surface; but still the result that the
maximum temperature beneath the crust in general attains a value
no higher than 270° C. is hardly tenable. We conclude, then, that
some other source of heat exists beneath. This may be radioactive
in origin and may be easily accounted for if the radioactive
materials are more sparsely distributed at the base of the upper
crust. Or, again, the heat may be primeval or original heat,
still escaping from a cooling world. For our present purpose it
does not much matter which view

127

we adopt. But we must recognise that the calculated depth of 19
kilometres of crust, possessing the average radioactivity of the
surface, is excessive; for, in fact, we are compelled by the
facts to recognise that some other source of heat exists
beneath.

If the observed surface gradient of temperature persisted
uniformly downwards, at some 35 kilometres beneath the surface
there would exist temperatures (of about 1000° C.) adequate to
soften basic rocks. It is probable, however, that the gradient
diminishes downwards, and that the level at which such
temperatures exist lies rather deeper than this. It is,
doubtless, somewhat variable according to local conditions; nor
can we at all approximate closely to an estimate of the depth at
which the fusion temperatures will be reached, for, in fact, the
existence of the radioactive layer very much complicates our
estimates. In what follows we assume the depth of softening to
lie at about 40 kilometres beneath the surface of the normal
crust; that is 25 miles down. It is to be observed that Prestwich
and other eminent geologists, from a study of the facts of
crust-folding, etc., have arrived at similar estimates.[1] As a
further assumption we are probably not far wrong if we assign to
the radioactive part of this crust a thickness of about 10 or 12
kilometres; _i.e._ six or seven miles. This is necessarily a
rough approximation only; but the conclusions at which

[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._

128

we shall arrive are reached in their essential features allowing
a wide latitude in our choice of data. We shall speak of this
part of the crust as the normal radioactive layer.

An important fact is evolved from the mathematical investigation
of the temperature conditions arising from the presence of such a
radioactive layer. It is found that the greatest temperature, due
to the radioactive heat everywhere evolved in the layer—_i.e._
the temperature at its base—is proportional to the square of the
thickness of the layer. This fact has a direct bearing on the
influence of radioactivity upon mountain elevation; as we shall
now find.

The normal radioactive layer of the Earth is composed of rocks
extending—as we assume—approximately to a depth of 12 kilometres
(7.5 miles). The temperature at the base of this layer due to the
heat being continually evolved in it, is, say, t₁°. Now, let us
suppose, in the trough of the geosyncline, and upon the top of
the normal layer, a deposit of, say, 10 kilometres (6.2 miles) of
sediments is formed during a long period of continental
denudation. What is the effect of this on the temperature at the
base of the normal layer depressed beneath this load? The total
thickness of radioactive rocks is now 22 kilometres. Accordingly
we find the new temperature t₂°, by the proportion t₁° : t₂° ::
12° : 22° That is, as 144 to 484. In fact, the temperature is more
than trebled. It is true we here assume the radioactivity of the
sediments

129

and of the normal crust to be the same. The sediments are,
however, less radioactive in the proportion of 4 to 3.
Nevertheless the effects of the increased thickness will be
considerable.

Now this remarkable increase in the temperature arises entirely
from the condition attending the radioactive heating; and
involves something _additional_ to the temperature conditions
determined by the mere depression and thickening of the crust as
in the Babbage-Herschel theory. The latter theory only involves a
_shifting_ of the temperature levels (or geotherms) into the
deposited materials. The radioactive theory involves an actual
rise in the temperature at any distance from the surface; so that
_the level in the crust at which the rocks are softened is nearer
to the surface in the geosynclines than it is elsewhere in the
normal crust_ (Pl. XV, p. 118).

In this manner the rigid part of the crust is reduced in
thickness where the great sedimentary deposits have collected. A
ten-kilometre layer of sediment might result in reducing the
effective thickness of the crust by 30 per cent.; a
fourteen-kilometre layer might reduce it by nearly 50 per cent.
Even a four-kilometre deposit might reduce the effective
resistance of the crust to compressive forces, by 10 per cent.

Such results are, of course, approximate only. They show that as
the sediments grow in depth there is a rising of the geotherm of
plasticity—whatever its true temperature may be—gradually
reducing the thickness of that part

130

of the upper crust which is bearing the simultaneously increasing
compressive stresses. Below this geotherm long-continued stress
resolves itself into hydrostatic pressure; above it (there is, of
course, no sharp line of demarcation) the crust accumulates
elastic energy. The final yielding and flexure occur when the
resistant cross-section has been sufficiently diminished. It is
probable that there is also some outward hydrostaitic thrust over
the area of rising temperature, which assists in determining the
upward throw of the folds.

When yielding has begun in any geosyncline, and the materials are
faulted and overthrust, there results a considerably increased
thickness. As an instance, consider the piling up of sediments
over the existing materials of the Alps, which resulted from the
compressive force acting from south to north in the progress of
Alpine upheaval. Schmidt of Basel has estimated that from 15 to
20 kilometres of rock covered the materials of the Simplon as now
exposed, at the time when the orogenic forces were actively at
work folding and shearing the beds, and injecting into their
folds the plastic gneisses coming from beneath.[1] The lateral
compression of the area of deposition of the Laramide, already
referred to, resulted in a great thickening of the deposits. Many
other cases might be cited; the effect is always in some degree
necessarily produced.