As we do not know the source or nature of the stimulus
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responsible for cell division we cannot give a molecular accountof death in the higher organisms. However we shall now see that,philosophically, we are entitled to consider reproduction as asaving factor in this case also; and to regard the death of theindividual much as we regard the fall of the leaf from the tree:_i.e._ as the cessation of an outgrowth from a developmentextending from the past into the future. The phenomena of old ageand natural death are, in short, not at variance with theprogressive activity of the organism. We perceive this when wecome 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 "deathis not a primary necessity but that it has been secondarilyacquired by adaptation." The cell was not inherently limited inits number of cell-generations. The low unicellular organisms arepotentially immortal, the higher multicellular forms withwell-differentiated organs contain the germs of death withinthemselves.
He finds the necessity of death in its utility to the species.Long life is a useless luxury. Early and abundant reproduction isbest for the species. An immortal individual would graduallybecome injured and would be valueless or even harmful to thespecies by taking the place of those that are sound. Hencenatural selection will shorten life.
[1] See his _Biological Memoirs._ Oxford, 1888.
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Weismann contends against the transmission of acquired charactersas being unproved.[1] He bases the appearance of death onvariations in the reproductive cells, encouraged by the ceaselessaction of natural selection, which led to a differentiation intoperishable somatic cells and immortal reproductive cells. Thetime-limit of any particular organism ultimately depends upon thenumber of somatic cell-generations and the duration of eachgeneration. These quantities are "predestined in the germ itself"which gives rise to each individual. "The existence of immortalmetazoan organisms is conceivable," but their capacity forexistence is influenced by conditions of the external world; thisrenders necessary the process of adaptation. In fact, in thedifferentiation of somatic from reproductive cells, material wasprovided upon which natural selection could operate to shorten orto lengthen the life of the individual in accordance with theneeds of the species. The soma is in a sense "a secondaryappendage of the real bearer of life—the reproductive cells." Thesomatic cells probably lost their immortal qualities, on thisimmortality becoming useless to the species. Their mortality mayhave been a mere consequence of their differentiation (loc. cit.,p. 140), itself due to natural selection. "Natural death wasnot," in fact, "introduced from absolute intrinsic necessityinherent in the nature of living matter, but on grounds ofutility,
[1] Biological Memoirs, p. 142.
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that is from necessities which sprang up, not from the generalconditions of life, but from those special conditions whichdominate the life of multicellular organisms."
On the inherent immortality of life, Weismann finally states:"Reproduction is, in truth, an essential attribute of livingmatter, just as the growth which gives rise to it.... Life iscontinuous, and not periodically interrupted: ever since itsfirst appearance upon the Earth in the lowest organism, it hascontinued without break; the forms in which it is manifest havealone undergone change. Every individual alive today—even thehighest—is to be derived in an unbroken line from the first andlowest forms." [1]
At the present day the view is very prevalent that the soma ofhigher organisms is, in a sense, but the carrier for a period ofthe immortal reproductive cells (Ray Lankester)[2]—an appendagedue to adaptation, concerned in their supply, protection, andtransmission. And whether we regard the time-limit of itsfunctions as due to external constraints, recurrently acting tilltheir effects become hereditary, or to variations more directlyof internal origin, encouraged by natural selection, we see inold age and death phenomena ultimately brought about in obedienceto the action of an environment. These are not inherent in theproperties of living matter. But, in spite
[1] Loc. cit., p. 159
[2] Geddes and Thomson, The Evolution of Sex, chap. xviii.
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of its mortality, the body remains a striking manifestation ofthe progressiveness of the organism, for to this it must beascribed. To it energy is available which is denied to theprotozoon. Ingenious adaptations to environment are moreespecially its privilege. A higher manifestation, however, waspossible, and was found in the development of mind. This, too, isa servant of the cell, as the genii of the lamp. Through itenergy is available which is denied to the body. This is themasterpiece of the cell. Its activity dates, as it were, but fromyesterday, and today it inherits the most diverse energies of theEarth.
Taking this view of organic succession, we may liken theindividual to a particle vibrating for a moment and then comingto rest, but sweeping out in its motion one wave in thecontinuous organic vibration travelling from the past into thefuture. But as this vibration is one spreading with increasedenergy from each vibrating particle, its propagation involves acontinual accelerated inflow of energy from the surroundingmedium, a dynamic condition unknown in periodic effectstransmitted by inanimate actions, and, indeed, marking thefundamental difference between the dynamic attitudes of theanimate and inanimate.
We can trace the periodic succession of individuals on a diagramof activity with some advantage. Considering, first, the case ofthe unicellular organism reproducing by subdivision and recallingthat conditions, definite and inevitable, oppose a limit to therate of growth, or, for our
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present purpose, rate of consumption of energy, we proceed asfollows:
{Fig. 1}
Along a horizontal axis units of time are measured; along avertical axis units of energy. Then the life-history of theamoeba, for example, appears as a line such as A in Fig. 1.During the earlier stages of its growth the rate of absorption ofenergy is small; so that in the unit interval of time, t, thesmall quantity of energy, e1, is absorbed. As life advances, theactivity of the organism augments, till finally this rate attainsa maximum, when e2 units of energy are consumed in the unit oftime.[1]
[1] Reference to p. 76, where the organic system is treated aspurely mechanical, may help readers to understand what isinvolved 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 ofits mechanism, affects the principles which justify the diagram.
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On this diagram reproduction, on the part of the organism, isrepresented by a line which repeats the curvature of the parentorganism originating at such a point as P in the path of thelatter, when the rate of consumption of energy has becomeconstant. The organism A has now ceased to act as a unit. Theproducts of fission each carry on the vital development of
{Fig. 2}
the species along the curve B, which may be numbered (2), tosignify that it represents the activity of two individuals, andso on, the numbering advancing in geometrical progression. Theparticular curvature adopted in the diagram is, of course,imaginary; but it is not of an indeterminate nature. Its coursefor any species is a characteristic of fundamental physicalimportance, regarding the part played in nature by the particularorganism.
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In Fig. 2 is represented the path of a primitive multicellularorganism before the effects of competition produced or fosteredits mortality. The lettering of Fig. 1 applies; the successivereproductive acts are marked P1, P2; Q1, Q2, etc., in the pathsof the successive individuals.
{Fig. 3}
The next figure (Fig. 3) diagrammatically illustrates death inorganic history. The path ever turns more and more from the axisof energy, till at length the point is reached when no moreenergy is available; a tangent to the curve at this point is atright angles to the axis of energy and parallel to the time axis.The death point is reached, and however great a length we measurealong the axis of time, no further consumption of energy is
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indicated by the path of the organism. Drawing the line beyondthe death point is meaningless for our present purpose.
It is observable that while the progress of animate nature findsits representation on this diagram by lines sloping _upwards_ fromleft to right, the course of events in inanimate nature—forexample, the history of the organic configuration after death, or
{Fig. 4}
the changes progressing—let us say, in the solar system, or inthe process of a crystallisation, would appear as lines slopingdownwards from left to right.
Whatever our views on the origin of death may be, we have torecognise a periodicity of functions in the life-history of thesuccessive individuals of the present day; and whether or not wetrace this directly or indirectly to
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a sort of interference with the rising wave of life, imposed bythe activity of a series of derived units, each seeking energy,and in virtue of its adaptation each being more fitted to obtainit than its predecessor, or even leave the idea of interferenceout of account altogether in the origination or perpetuation ofdeath, the truth of the diagram (Fig. 4) holds in so far as itmay be supposed to graphically represent the dynamic history ofthe individual. The point chosen on the curve for the originationof a derived unit is only applicable to certain organisms, manyreproducing at the very close of life. A chain of units aresupposed here represented.[1]
THE LENGTH OF LIFE
If we lay out waves as above to a common scale of time fordifferent species, the difference of longevity is shown in thegreater or less number of vibrations executed in a given time,_i.e._ in greater or less "frequency." We cannot indeed draw thecurvature correctly, for this would necessitate a knowledge whichwe have not of the activity of the organism at different periodsof its life-history, and so neither can we plot the direction ofthe organic line of propagation with respect to the
[1] Projecting upon the axes of time and energy any one completevibration, as in Fig. 4, the total energy consumed by theorganism during life is the length E on the axis of energy, andits period of life is the length T on the time-axis. The meanactivity is the quotient E/T.
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axes of reference as this involves a knowledge of the meanactivity.[1]
The group of curves which follow, relating to typical animalspossessing very different activities (Fig. 5), are thereforeentirely diagrammatic, except in respect to the approximate
{Fig. 5}
longevity of the organisms. (1) might represent an animal of thelength of life and of the activity of Man; (2), on the same scaleof longevity,
[1] In the relative food-supply at various periods of life thecurvature is approximately determinable.
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one of the smaller mammals; and (3), the life-history of a coldblooded animal living to a great age; _e.g._ certain of thereptilia.
It is probable, that to conditions of structural development,under the influence of natural selection, the question of longeror shorter life is in a great degree referable. Thus, developmentalong lines of large growth will tend to a slow rate ofreproduction from the simple fact that unlimited energy to supplyabundant reproduction is not procurable, whatever we may assumeas to the strength or cunning exerted by the individual in itsefforts to obtain its supplies. On the other hand, developmentalong lines of small growth, in that reproduction is less costly,will probably lead to increased rate of reproduction. It is, infact, matter of general observation that in the case of largeranimals the rate of reproduction is generally slower than in thecase of smaller animals. But the rate of reproduction might beexpected to have an important influence in determining theparticular periodicity of the organism. Were we to depict in thelast diagram, on the same time-scale as Man, the vibrations ofthe smaller and shorter-lived living things, we would see but astraight line, save for secular variations in activity,representing the progress of the species in time: the tinythrills of its units lost in comparison with the yet brief periodof Man.
The interdependence of the rate of reproduction and
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the duration of the individual is, indeed, very probably revealedin the fact that short-lived animals most generally reproducethemselves rapidly and in great abundance, and vice versa. Inmany cases where this appears contradicted, it will be found thatthe young are exposed to such dangers that but few survive (_e.g._many of the reptilia, etc.), and so the rate of reproduction isactually slow.
Death through the periodic rigour of the inanimate environmentcalls forth phenomena very different from death introduced orfavoured by competition. A multiplicity of effects simulative ofdeath occur. Organisms will, for example, learn to meet veryrigorous conditions if slowly introduced, and not permanent. Atransitory period of want can be tided over by contrivance. Thelily withdrawing its vital forces into the bulb, protected fromthe greatest extremity of rigour by seclusion in the Earth; thetrance of the hibernating animal; are instances of suchcontrivances.
But there are organisms whose life-wave truly takes up theperiodicity of the Earth in its orbit. Thus the smaller animalsand plants, possessing less resources in themselves, die at theapproach of winter, propagating themselves by units which,whether egg or seed, undergo a period of quiescence during theseason of want. In these quiescent units the energy of theorganism is potential, and the time-energy function is inabeyance. This condition is, perhaps, foreshadowed in theencyst-
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ment of the amoeba in resistance to drought. In most cases ofhibernation the time-energy function seems maintained at a lossof potential by the organism, a diminished vital consumption ofenergy being carried on at the expense of the stored energy ofthe tissues. So, too, even among the largest organisms there willbe a diminution of activity periodically inspired byclimatological conditions. Thus, wholly or in part, the activityof organisms is recurrently affected by the great energy—tidesset up by the Earth's orbital motion.
{Fig. 6}
Similarly in the phenomenon of sleep the organism responds to theEarth's axial periodicity, for in the interval of night a periodof impoverishment has to be endured. Thus the diurnal waves ofenergy also meet a response in the organism. These tides andwaves of activity would appear as larger and smaller ripples
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on the life-curve of the organism. But in some, in which life anddeath are encompassed in a day, this would not be so; and for theannual among plants, the seed rest divides the waves with linesof no activity (Fig. 6).
Thus, finally, we regard the organism as a dynamic phenomenonpassing through periodic variations of intensity. The materialsystems 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 consumptionof energy is approximately the same; the functions, too, whichdemand and expend the energy are of similar nature.
That the rhythm of these events is ultimately based on harmony inthe configuration and motion of the molecules within the germseems an unavoidable conclusion. In the life of the individualrhythmic dynamic phenomena reappear which in some cases have nolonger a parallel in the external world, or under conditions whenthe individual is no longer influenced by these externalconditions.,, In many cases the periodic phenomena ultimately dieout under new influences, like the oscillations of a body in aviscous medium; in others when they seem to be more deeply rootedin physiological conditions they persist.
The "length of life is dependent upon the number
[1] The _Descent of Man._
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of generations of somatic cells which can succeed one another inthe course of a single life, and furthermore the number as wellas the duration of each single cell-generation is predestined inthe germ itself."[1]
Only in the vague conception of a harmonising or formativestructural influence derived from the germ, perishing in eachcell from internal causes, but handed from cell to cell till theformative influence itself degrades into molecular discords, doesit seem possible to form any physical representation of thesuccessive events of life. The degradation of the molecularformative influence might be supposed involved in its frequenttransference according to some such dynamic actions as occur ininanimate nature. Thus, ultimately, to the waste within the cell,to the presence of a force retardative of its perpetual harmonicmotions, the death of the individual is to be ascribed. Perhapsin protoplasmic waste the existence of a universal death shouldbe recognised. It is here we seem to touch inanimate nature; andwe are led back to a former conclusion that the organism in itsunconstrained state is to be regarded as a contrivance forevading the dynamic tendencies of actions in which lifelessmatter participates.[2]
[1] Weismann, _Life and Death; Biological Memoirs_, p. 146.
[2] In connection with the predestinating power and possiblecomplexity of the germ, it is instructive to reflect on the verygreat molecular population of even the smallest spores—givingrise to very simple forms. Thus, the spores of the unicellularSchizomycetes are estimated to dimensions as low as 1/10,000 of amillimetre 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 isprobable that such spores contain some 500,000 molecules, whileone hundred molecules range along a diameter.
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THE NUMERICAL ABUNDANCE OF LIFE
We began by seeking in various manifestations of life a dynamicprinciple sufficiently comprehensive to embrace its very variousphenomena. This, to all appearance, found, we have been led toregard life, to a great extent, as a periodic dynamic phenomenon.Fundamentally, in that characteristic of the contrivance, whichleads it to respond favourably to transfer of energy, itsenormous extension is due. It is probable that to its instabilityits numerical abundance is to be traced—for this, necessitatingthe continual supply of all the parts already formed, renderslarge, undifferentiated growth, incompatible with the limitedsupplies of the environment. These are fundamental conditions ofabundant life upon the Earth.
Although we recognise in the instability of living systems theunderlying reason for their numerical abundance, secondaryevolutionary causes are at work. The most important of these isthe self-favouring nature of the phenomenon of reproduction. Thusthere is a tendency not only to favour reproductiveness, butearly reproductiveness, in the form of one prolificreproductive.
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act, after which the individual dies.[1] Hence the wavelength ofthe species diminishes, reproduction is more frequent, andcorrespondingly greater numbers come and go in an interval oftime.
Another cause of the numerical abundance of life exists, asalready stated, in the conditions of nourishment. Energy is morereadily conveyed to the various parts of the smaller mass, andhence the lesser organisms will more actively functionate; andthis, as being the urging dynamic attitude, as well as that mostgenerally favourable in the struggle, will multiply and favoursuch forms of life. On the other hand, however, these forms willhave less resource within themselves, and less power ofendurance, so that they are only suitable to fairly uniformconditions of supply; they cannot survive the long continued wantof winter, and so we have the seasonal abundance of summer. Onlythe larger and more resistant organisms, whether animal orvegetable, will, in general, populate the Earth from year toyear. From this we may conclude that, but for the seasonalenergy-tides, the development of life upon the globe had gonealong very different lines from those actually followed. It is,indeed, possible that the evolution of the larger organisms wouldnot have occurred; there would have been no vacant place fortheir development, and a being so endowed as Man could hardly
[1] Weismann, _The Duration of Life._
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have been evolved. We may, too, apply this reasoning elsewhere,and regard as highly probable, that in worlds which are withoutseasonal influences, the higher developments of life have notappeared; except they have been evolved under other conditions,when they might for a period persist. We have, indeed, only topicture to ourselves what the consequence of a continuance ofsummer would be on insect life to arrive at an idea of theantagonistic influences obtaining in such worlds to the survivalof larger organisms.
It appears that to the dynamic attitude of life in the firstplace, and secondarily to the environmental conditions limitingundifferentiated growth, as well as to the action of heredity intransmitting the reproductive qualities of the parent to theoffspring, the multitudes of the pines, and the hosts of ants,are to be ascribed. Other causes are very certainly at work, butthese, I think, must remain primary causes.
We well know that the abundance of the ants and pines is not atithe of the abundance around us visible and invisible. It is avain endeavour to realise the countless numbers of ourfellow-citizens upon the Earth; but, for our purpose, therestless ants, and the pines solemnly quiet in the sunshine, haveserved as types of animate things. In the pine the gates of theorganic have been thrown open that the vivifying river of energymay flow in. The ants and the butterflies sip for a brief momentof its waters, and again vanish into the
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inorganic: life, love and death encompassed in a day.
Whether the organism stands at rest and life comes to it on thematerial currents of the winds and waters, or in the vibratoryenergy of the æther; or, again, whether with restless craving ithurries hither and thither in search of it, matters nothing. Theone principle—the accelerative law which is the law of theorganic—urges all alike onward to development, reproduction anddeath. But although the individual dies death is not the end; forlife is a rhythmic phenomenon. Through the passing ages the wavesof life persist: waves which change in their form and in thefrequency to which they are attuned from one geologic period tothe next, but which still ever persist and still ever increase.And in the end the organism outlasts the generations of thehills.
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THE BRIGHT COLOURS OF ALPINE FLOWERS[1]
IT is admitted by all observers that many species of floweringplants growing on the higher alps of mountainous regions displaya more vivid and richer colour in their bloom than is displayedin the same species growing in the valleys. That this is actuallythe case, and not merely an effect produced upon the observer bythe scant foliage rendering the bloom more conspicuous, has beenshown by comparative microscopic examination of the petals ofspecies growing on the heights and in the valleys. Suchexamination has revealed that in many cases pigment granules aremore 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 andGeranium sylvaticum; while in Rosa alpina and Erigeron alpinus nodifference is observable.[2]
In the following cases a difference of intensity of colour is,according to Kerner ("Pflanzenleben," 11. 504), especiallynoticeable:— _Agrostemma githago, Campanula
[1] _Proc. Royal Dublin Society_, 1893.
[2] G. Bonnier, quoted by De Varigny, _Experimental Evolution_,p. 55.
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pusilla, Dianthus inodorus (silvestris), Gypsophila repens, Lotuscorniculatus, Saponaria ocymoides, Satureja hortensis, Taraxacummofficinale, Vicia cracca, and Vicia sepium._
To my own observation this beautiful phenomenon has alwaysappeared most obvious and impressive. It appears to have struckmany unprofessional observers. Helmholtz offers the explanationthat the vivid colours are the result of the brighter sunlight ofthe heights. It has been said, too, that they are the directchemical effects of a more highly ozonized atmosphere. The latterexplanation I am unable to refer to its author. The followingpages contain a suggestion on the matter, which occurred to mewhile touring, along with Henry H. Dixon, in the Linthal districtof Switzerland last summer.[1]
If the bloom of these higher alpine flowers is especiallypleasing to our own æsthetic instincts, and markedly conspicuousto us as observers, why not also especially attractive andconspicuous to the insect whose mission it is to wander fromflower to flower over the pastures? The answer to this questioninvolves the hypothesis I would advance as accounting for thebright colours of high-growing individuals. In short, I believe asatisfactory explanation is to be found in the conditions ofinsect life in the higher alps.
In the higher pastures the summer begins late and
[1] The summer of 1892.
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closes early, and even in the middle of summer the day closes inwith extreme cold, and the cold of night is only dispelled whenthe sun is well up. Again, clouds cover the heights when all isclear below, and cold winds sweep over them when there is warmthand shelter in the valleys. With these rigorous conditions thepollinating insects have to contend in their search for food, andthat when the rival attractions of the valleys below are so many.I believe it is these rigorous conditions which are indirectlyresponsible for the bright colours of alpine flowers. For suchconditions will bring about a comparative scarcity of insectactivity on the heights; and a scarcity or uncertainty in theaction of insect agency in effecting fertilization will intensifythe competition to attract attention, and only the brightestblooms 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 inArcady_ the theory that there is a natural selective causefostering the bright blooms of alpines. The selective cause is,however, by him referred to the greater abundance of butterflyrelatively to bee fertilizers. The former, he says, display moreæsthetic instinct than bees. In the valley the bees secure thefertilization of all. I may observe that upon the Fridolins Alpall the fertilizers we observed were bees. I have always foundbutterflies very scarce at altitudes of 7,000 to 8,000 feet. Thealpine bees are very light in body, like our hive bee, and I donot think rarefaction of the atmosphere can operate to hinder itsascent to the heights, as Grant Allen suggests. The observationson the death-rate of bees and butterflies on the glacier, to bereferred to presently, seem to negative such a hypothesis, and toshow that a large preponderance of bees over butterflies maketheir way to the heights.
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brightest will be the fittest, and this condition, along with theinfluence of heredity, will encourage a race of vivid flowers. Onthe other hand, the more scant and uncertain root supply, and thesevere atmospheric conditions, will not encourage the grosserstruggle for existence which in the valleys is carried on soeagerly between leaves and branches—the normal offensive anddefensive weapons of the plant—and so the struggle becomesrefined into the more æsthetic one of colour and brightnessbetween flower and flower. Hence the scant foliage and vividbloom would be at once the result of a necessary economy, and aresort to the best method of securing reproduction under thecircumstances of insect fertilizing agency. Or, in other words,while the luxuriant growth is forbidden by the conditions, andthus methods of offence and defence, based upon vigorousdevelopment, reduced in importance, it would appear that thestruggle is mainly referred to rivalry for insect preference. Itis probable that this is the more economical manner of carryingon the contest.
In the valleys we see on every side the struggle between thevegetative organs of the plant; the soundless battle among theleaves and branches. The blossom here is carried aloft on aslender stem, or else, taking but a secondary part in thecontest, it is relegated to obscurity (P1. XII.). Further up onthe mountains, where the conditions are more severe and thesupplies less abundant, the leaf and branch assume lesserdimensions, for they are costly weapons to provide and theelements are unfriendly
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to their existence (Pl. XIII.). Still higher, approaching theclimatic limit of vegetable life, the struggle for existence ismainly carried on by the æsthetic rivalry of lowly butconspicuous blossoms.
As regards the conditions of insect life in the higher alps, itcame to my notice in a very striking manner that vast numbers ofsuch bees and butterflies as venture up perish in the cold ofnight time. It appears as if at the approach of dusk these areattracted by the gleam of the snow, and quitting the pastures,lose themselves upon the glaciers and firns, there to die inhundreds. Thus in an ascent of the Tödi from the Fridolinshüte wecounted in the early dawn sixty-seven frozen bees, twenty-ninedead butterflies, and some half-dozen moths on the BifertenGlacier and Firn. These numbers, it is to be remembered, onlyincluded those lying to either side of our way over the snow, sothat the number must have mounted up to thousands when integratedover the entire glacier and firn. Approaching the summit nonewere found. The bees resembled our hive bee in appearance, thebutterflies resembled the small white variety common in ourgardens, which has yellow and black upon its wings. One largemoth, striped across the abdomen, and measuring nearly two inchesin length of body, was found. Upon our return, long after thesun's rays had grown strong, we observed some of the butterfliesshowed signs of reanimation. We descended so quickly to avoid theinconvenience of the soft snow that we had time for no
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close observation on the frozen bees. But dead bees are commonobjects 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 followingnote:
"Late Flowering Plants.—While we write, the ivy is in flower, andbees, wasps, and flies are jostling each other and struggling tofind standing-room on the sweet-smelling plant. How great must bethe advantage obtained by this plant through its exceptionalhabit of flowering in the late autumn, and ripening its fruit inthe spring. To anyone who has watched the struggle to approachthe ivy-blossom at a time when nearly all other plants are bare,it is evident that, as far as transport of pollen andcross-fertilization go, the plant could not flower at a moresuitable time. The season is so late that most other plants areout of flower, but yet it is not too late for many insects to bebrought out by each sunny day, and each insect, judging by itsbehaviour, must be exceptionally hungry.
"Not only has the ivy the world to itself during its floweringseason, but it delays to ripen its seed till the spring, a timewhen most other plants have shed their seed, and most ediblefruits have been picked by the birds. Thus birds wanting fruit inthe spring can obtain little but ivy, and how they appreciate theivy berry is evident
[1] For December, 1892, vol. i., p. 730.
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by the purple stains everywhere visible within a short distanceof the bush."
These remarks suggest that the ivy adopts the converse attitudetowards its visitors to that forced upon the alpine flower. Theivy bloom is small and inconspicuous, but then it has the seasonto itself, and its inconspicuousness is no disadvantage, _i.e._if one plant was more conspicuous than its neighbours, it wouldnot have any decided advantage where the pollinating insect isabundant and otherwise unprovided for. Its dark-green berries inspring, which I would describe as very inconspicuous, have asimilar advantage in relation to the necessities of bird life.
The experiments of M. C. Flahault must be noticed. Thisnaturalist grew seeds of coloured flowers which had ripened inParis, part in Upsala, and part in Paris; and seed which hadripened in Upsala, part at Paris, and part at Upsala. The flowersopening in the more northern city were in most cases thebrighter.[1] If this observation may be considered indisputable,as appears to be the case, the question arises, Are we to regardthis as a direct effect of the more rigorous climate upon thedevelopment of colouring matter on the blooms opening at Upsala?If we suppose an affirmative answer, the theory of direct effectby sun brightness must I think be abandoned. But I venture tothink that the explanation of the Upsala
[1] Quoted by De Varigny, _Experimental Evolution_, p. 56.
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experiment is not to be found in direct climatic influence uponthe colour, but in causes which lie deeper, and involve somefactors deducible from biological theory.
The organism, as a result of the great facts of heredity and ofthe survival of the fittest, is necessarily a system whichgathers experience with successive generations; and the principallesson ever being impressed upon it by external events iseconomy. Its success depends upon the use it makes of itsopportunities for the reception of energy and the economyattained in disposing of what is gained.
With regard to using the passing opportunity the entire seasonaldevelopment of life is a manifestation of this attitude, and thefleetness, agility, etc., of higher organisms are developments inthis direction. The higher vegetable organism is not locomotory,save in the transferences of pollen and seed, for its food comesto it, and the necessary relative motion between food andorganism is preserved in the quick motion of radiated energy fromthe sun and the slower motion of the winds on the surface of theearth. But, even so, the vegetable organism must stand ever readyand waiting for its supplies. Its molecular parts must be readyto seize the prey offered to it, somewhat as the waiting spiderthe fly. Hence, the plant stands ready; and every cloud withmoving shadow crossing the fields handicaps the shaded to thebenefit of the unshaded plant in the adjoining field. The openbloom
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is a manifestation of the generally expectant attitude of theplant, but in relation to reproduction.
As regards economy, any principle of maximum economy, where manyfunctions have to be fulfilled, will, we may very safely predict,involve as far as possible mutual helpfulness in the processesgoing on. Thus the process of the development towards meeting anyparticular external conditions, A, suppose, will, if possible,tend to forward the development towards meeting conditions B; sothat, in short, where circumstances of morphology and physiologyare favourable, the ideally economical system will be attainedwhen in place of two separate processes, a, ß, the one process y,cheaper than a + ß, suffices to advance developmentsimultaneously in both the directions A and B. The economy is asobvious as that involved in "killing two birds with the onestone"—if so crude a simile is permissible—and it is to beexpected that to foster such economy will be the tendency ofevolution in all organic systems subjected to restraints as thosewe are acquainted with invariably are.
Such economy might be simply illustrated by considering the caseof a reservoir of water elevated above two hydraulic motors, sothat the elevated mass of water possessed gravitationalpotential. The available energy here represents the stored-upenergy in the organism. How best may the water be conveyed to thetwo motors [the organic systems reacting towards conditions A andB] so
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that as little energy as possible is lost in transit? If themotors are near together it is most economical to use the oneconduit, which will distribute the requisite supply of water toboth. If the motors are located far asunder it will be mosteconomical to lay separate conduits. There is greatest economy inmeeting a plurality of functions by the same train ofphysiological processes where this is consistent with meetingother demands necessitated by external or internal conditions.
But an important and obvious consequence arises in the supply ofthe two motors from the one conduit. We cannot work one motorwithout working the other. If we open a valve in the conduit bothmotors start into motion and begin consuming the energy stored inthe tank. And although they may both under one set of conditionsbe doing useful and necessary work, in some other set ofconditions it may be needless for both to be driven.
This last fact is an illustration of a consideration which mustenter into the phenomenon which an eminent biologist speaks of asphysiological or unconscious "memory,"[1] For the development ofthe organism from the ovum is but the starting of a train ofinterdependent events of a complexity depending upon theexperience of the past.
[1] Ewald Hering, quoted by Ray Lankaster, _The Advancement ofScience_, p. 283.
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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 toassociate certain groups of conditions. Thus, in the case inpoint, climatic rigour and scarcity of pollinating agency willever be associated; and in the long experience of the past themost economical physiological attitude towards both is, we maysuppose, adopted; so that the presence of one condition excitesthe apparent unconscious memory of the other. In reality theprocess of meeting the one condition involves the process anddevelopment for meeting the other.
And this consideration may be extended very generally to suchorganisms as can survive under the same associated naturalconditions, for the history of evolution is so long, and thepower of locomotion so essential to the organism at some periodin its life history, that we cannot philosophically assume alocal history for members of a species even if widely severedgeographically at the present day. At some period in the pastthen, it is very possible that the individuals today thriving atParis, acquired the experience called out at Upsala. Theperfection of physiological memory inspires no limit to the dateat which this may have occurred—possibly the result of asuccession of severe seasons at Paris; possibly the result ofmigrations —and the seed of many flowering plants possess meansof migration only inferior to those possessed by the flying andswimming animals. But, again, possibly the experi-
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ence was acquired far back in the evolutionary history of theflower.[1]
But a further consideration arises. Not only at each moment inthe life of the individual must maximum income and most judiciousexpenditure be considered, but in its whole life history, andeven over the history of its race, the efficiency must tend to bea maximum. This principle is even carried so far that whennecessary it leads to the death of the individual, as in the caseof those organisms which, having accomplished the reproductiveact, almost immediately expire. This view of nature may berepellent, but it is, nevertheless, evident that we are parts ofa system which ruthlessly sacrifices the individual on generalgrounds of economy. Thus, if the curve which defines the meanrate of reception of energy of all kinds at different periods inthe life of the organism be opposed by a second curve, drawnbelow the axis along which time is measured, representing themean 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 ofcleistogamic plants (_e.g._ Viola), are examples of unconsciousmemory, or unconscious "association of ideas" leading to thedevelopment of organs now functionless. The _Pontederia crassipes_of the Amazon, which develops its floating bladders when grown inwater, but aborts them rapidly when grown on land, and seems toretain this power of adaptation to the environment for anindefinite period of time, must act in each case upon anunconscious memory based upon past experience. Many other casesmight be cited.
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physiologically dependent on the former, must be of such a naturefrom its origin to its completion in death, that the condition isrealized of the most economical rate of expenditure at eachperiod of life.[1] The rate of expenditure of energy at anyperiod of life is, of course, in such a curve defined by theslope of the curve towards the axis of time at the period inquestion; but this particular slope _must be led to by a previouspart of the curve, and involves its past and future course to avery great extent_.
{Fig. 7}
There will, therefore, be impressed upon theorganism by the factors of evolution a unified course ofeconomical expenditure completed only by its death, and whichwill give to the developmental progress of the individual itsprophetic 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_.
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when it is done with vitality, for that preparation for momentousorganic events which is in progress throughout the entire courseof development; and to the economy involved in the welding ofphysiological processes for the phenomenon of physiologicalmemory, wherein we see reflected, as it were, in the developmentof the organism, the association of inorganic restraintsoccurring in nature which at some previous period impresseditself upon the plastic organism. We may picture the seedling atUpsala, swayed by organic memory and the inherited tendency to aneconomical preparation for future events, gradually developingtowards the æsthetic climax of its career. In some such manneronly does it appear possible to account for the propheticdevelopment of organisms, not alone to be observed in the alpineflowers, but throughout nature.
And thus, finally, to the effects of natural selection and toactions defined by general principles involved in biology, Iwould refer for explanation of the manner in which flowers on theAlps develop their peculiar beauty.
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MOUNTAIN GENESIS
OUR ancestors regarded mountainous regions with feelings ofhorror, mingled with commiseration for those whom an unkindlydestiny had condemned to dwell therein. We, on the other hand,find in the contemplation of the great alps of the Earth suchpeaceful and elevated thoughts, and such rest to our souls, thatit is to those very solitudes we turn to heal the wounds of ife.It is difficult to explain the cause of this very different pointof view. It is probably, in part, to be referred to that cloud ofsuperstitious horror which, throughout the Middle Ages, peopledthe solitudes with unknown terrors; and, in part, to theasceticism which led the pious to regard the beauty and joy oflife as snares to the soul's well-being. In those eternalsolitudes where the overwhelming forces of Nature are most inevidence, an evil principle must dwell or a dragon's dreadfulbrood must find a home.
But while in our time the aesthetic aspect of the hills appealsto all, there remains in the physical history of the mountainsmuch that is lost to those who have not shared in the scientificstudies of alpine structure and genesis. They lose a past historywhich for interest com-
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petes with anything science has to tell of the changes of theEarth.
Great as are the physical features of the mountains compared withthe works of Man, and great as are the forces involved comparedwith those we can originate or control, the loftiest ranges aresmall contrasted with the dimensions of the Earth. It is well tobear this in mind. I give here (Pl. XV.) a measured drawingshowing a sector cut from a sphere of 50 cms. radius; so much ofit as to exhibit the convergence of its radial boundaries whichif prolonged will meet at the centre. On the same scale as theradius the diagram shows the highest mountains and the deepestocean. The average height of the land and the average depth ofthe ocean are also exhibited. We see how small a movement of thecrust the loftiest elevation of the Himalaya represents and whata little depression holds the ocean.
Nevertheless, it is not by any means easy to explain the genesisof those small elevations and depressions. It would lead us farfrom our immediate subject to discuss the various theoreticalviews which have been advanced to account for the facts. The ideathat mountain folds, and the lesser rugosities of the Earth'ssurface, arose in a wrinkling of the crust under the influence ofcooling and skrinkage of the subcrustal materials, is held bymany eminent geologists, but not without dissent from others.
The most striking observational fact connected with mountainstructure is that, without exception, the ranges
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of the Earth are built essentially of sedimentary rocks: that isof rocks which have been accumulated at some remote past timebeneath the surface of the ocean. A volcanic core there maysometimes be—probably an attendant or consequence of theuplifting—or a core of plutonic igneous rocks which has arisenunder the same compressive forces which have bowed and arched thestrata from their original horizontal position. It is notuncommon to meet among unobservant people those who regard allmountain ranges as volcanic in origin. Volcanoes, however, do notbuild mountain ranges. They break out as more or less isolatedcones or hills. Compare the map of the Auvergne with that ofSwitzerland; the volcanoes of South Italy with the Apennines.Such great ranges as those which border with triple walls thewest coast of North America are in no sense volcanic: nor are thePyrenees, the Caucasus, or the Himalaya. Volcanic materials arepoured out from the summits of the Andes, but the range itself isbuilt up of folded sediments on the same architecture as theother great ranges of the Earth.
Before attempting an explanation of the origin of the mountainswe must first become more closely acquainted with the phenomenaattending mountain elevation.
At the present day great accumulations of sediment are takingplace along the margins of the continents where the rivers reachthe ocean. Thus, the Gulf of Mexico receiving the sediment of theMississippi and Rio Grande;
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the northeast coast of South America receiving the sediments ofthe Amazons; the east coast of Asia receiving the detritus of theChinese rivers; are instances of such areas of deposition. Yearby year, century by century, the accumulation progresses, and asit grows the floor of the sea sinks under the load. Of theyielding of the crust under the burthen of the sediments we areassured; for otherwise the many miles of vertically piled stratawhich are uplifted to our view in the mountains, never could havebeen deposited in the coastal seas of the past. The flexure andsinking of the crust are undeniable realities.
Such vast subsiding areas are known as geosynclines. From theaccumulated sediments of the geosynclines the mountain ranges ofthe past have in every case originated; and the mountains of thefuture will assuredly arise and lofty ranges will stand where nowthe ocean waters close over the collecting sediments. Everymountain range upon the Earth enforces the certainty of thisprediction.
The mountain-forming movement takes place after a certain greatdepth of sediment is collected. It is most intense where thethickness of deposit is greatest. We see this when we examine thestructure of our existing mountain ranges. At either side wherethe sediments thin out, the disturbance dies away, till we findthe comparatively shallow and undisturbed level sediments whichclothe the continental surface.
Whatever be the connection between the deposition and
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the subsequent upheaval, _the element of great depth ofaccumulation seems a necessary condition and must evidently enteras a factor into the Physical Processes involved_. The mountainrange can only arise where the geosyncline is deeply filled bylong ages of sedimentation.
Dana's description of the events attending mountain building isimpressive:
"A mountain range of the common type, like that to which theAppalachians belong, is made out of the sedimentary formations ofa long preceding era; beds that were laid down conformably, andin succession, until they had reached the needed thickness; bedsspreading over a region tens of thousands of square miles inarea. The region over which sedimentary formations were inprogress in order to make, finally, the Appalachian range,reached from New York to Alabama, and had a breadth of 100 to 200miles, and the pile of horizontal beds along the middle was40,000 feet in depth. The pile for the Wahsatch Mountains was60,000 feet thick, according to King. The beds for theAppalachians were not laid down in a deep ocean, but in shallowwaters, where a gradual subsidence was in progress; and they atlast, when ready for the genesis, lay in a trough 40,000 feetdeep, filling the trough to the brim. It thus appears that epochsof mountain-making have occurred only after long intervals ofquiet in the history of a continent."[1]
[1] Dana, _Manual of Geology_, third edition, p. 794
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On the western side of North America the work ofmountain-building was, indeed, on the grandest scale. For longages and through a succession of geological epochs, sedimentationhad proceeded so that the accumulations of Palaeozoic andMesozoic times had collected in the geosyncline formed by theirown ever increasing weight. The site of the future Laramide rangewas in late Cretaceous times occupied by some 50,000 feet ofsedimentary deposits; but the limit had apparently been attained,and at this time the Laramide range, as well as its southerlycontinuation into the United States, the Rockies, had theirbeginning. Chamberlin and Salisbury[1] estimate that the heightof the mountains developed in the Laramide range at this time was20,000 feet, and that, owing to the further elevation which hassince taken place, from 32,000 to 35,000 feet would be theirpresent height if erosion had not reduced them. Thus on eitherside of the American continent we have the same forces at work,throwing up mountain ridges where the sediments had formerly beenshed into the ocean.
These great events are of a rhythmic character; the crust, as itwere, pulsating under the combined influences of sedimentationand denudation. The first involves downward movements under agathering load, and ultimately a reversal of the movement to oneof upheaval; the second factor, which throughout has been in
[1] Chamberlin and Salisbury, _Geology_, 1906, iii., 163.
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operation as creator of the sediments, then intervenes as anassailant of the newly-raised mountains, transporting theirmaterials again to the ocean, when the rhythmic action isrestored to its first phase, and the age-long sequence of eventsmust begin all over again.
It has long been inferred that compressive stress in the crustmust be a primary condition of these movements. The wvorkrequired to effect the upheavals must be derived from somepreexisting source of energy. The phenomenon—intrinsically one offolding of the crust—suggests the adjustment of the earth-crustto a lessening radius; the fact that great mountain-buildingmovements have simultaneously affected the entire earth iscertainly in favour of the view that a generally prevailing causeis at the basis of the phenomenon.
The compressive stresses must be confined to the upper few milesof the crust, for, in fact, the downward increase of temperatureand pressure soon confers fluid properties on the medium, andslow tangential compression results in hydrostatic pressurerather than directed stresses. Thus the folding visible in themountain 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 survivingpart of the original gravitational potential energy of the crustitself. It must be assumed that the crust in following downwardsthe shrinking subcrustal magma, develops immense compressivestresses in
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its materials, vast geographical areas being involved. Whenfolding at length takes place along the axis of the elongatedsyncline of deposition, the stresses find relief probably forsome hundreds of miles, and the region of folding now becomescompressed in a transverse direction. As an illustration, theLaramide range, according to Dawson, represents the reduction ofa surface-belt 50 miles wide to one of 25 miles. The marvelloustranslatory movements of crustal folds from south to northarising in the genesis of the Swiss Alps, which recent researchhas brought to light, is another example of these movements ofrelief, which continue to take place perhaps for many millions ofyears after they are initiated.
The result of this yielding of the crust is a buckling of thesurface which on the whole is directed upwards; but depressionalso is an attendant, in many cases at least, on mountainupheaval. Thus we find that the ocean floor is depressed into asyncline along the western coast of South America; a troughalways parallel to the ranges of the Andes. The downwarddeflection of the crust is of course an outcome of the samecompressive stresses which elevate the mountain.
The fact that the yielding of the crust is always situated wherethe sediments have accumulated to the greatest depth, has led toattempts from time to time of establishing a physical connexionbetween the one and the other. The best-known of these theoriesis that of Babbage and Herschel. This seeks the connexion in therise of the
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geotherms into the sinking mass of sediment and the consequentincrease of temperature of the earth-crust beneath. It will beunderstood that as these isogeotherms, or levels at which thetemperature is the same, lie at a uniform distance from thesurface all over the Earth, unless where special variations ofconductivity may disturb them, the introduction of materialpressed downwards from above must result in these materialspartaking of the temperature proper to the depth to which theyare depressed. In other words the geotherms rise into the sinkingsediments, always, however, preserving their former averagedistance from the surface. The argument is that as this processundoubtedly involves the heating up of that portion of the crustwhich the sediments have displaced downwards, the result must bea local enfeeblement of the crust, and hence these areas becomethose of least resistance to the stresses in the crust.
When this theory is examined closely, we see that it only amountsto saying that the bedded rocks, which have taken the place ofthe igneous materials beneath, as a part of the rigid crust ofthe Earth, must be less able to withstand compressive stress thanthe average crust. For there has been no absolute rise of thegeotherms, the thermal conductivities of both classes ofmaterials differing but little. Sedimentary rock has merely takenthe place of average crust-rock, and is subjected to the sameaverage temperature and pressure prevailing in the surroundingcrust. But are there any grounds for the
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assumption that the compressive resistance of a complex ofsedimentary rocks is inferior to one of igneous materials? Themetamorphosed siliceous sediments are among the strongest rocksknown as regards resistance to compressive stress; and iflimestones have indeed plastic qualities, it must be rememberedthat their average amount is only some 5 per cent. of the whole.Again, so far as rise of temperature in the upper crust mayaffect the question, a temperature which will soften an averageigneous rock will not soften a sedimentary rock, for the reasonthat the effect of solvent denudation has been to remove thosealkaline silicates which confer fusibility.
When, however, we take into account the radioactive content ofthe sediments the matter assumes a different aspect.
The facts as to the general distribution of radioactivesubstances at the surface, and in rocks which have come fromconsiderable depths in the crust, lead us to regard as certainthe widespread existence of heat-producing radioactive elementsin the exterior crust of the Earth. We find, indeed, in this factan explanation—at least in part—of the outflow of heatcontinually taking place at the surface as revealed by the risingtemperature inwards. And we conclude that there must be athickness of crust amounting to some miles, containing theradioactive elements.
Some of the most recent measurements of the quantities of radiumand thorium in the rocks of igneous origin—_e.g._ granites,syenites, diorites, basalts, etc., show that the
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radioactive heat continually given out by such rocks amounts toabout one millionth part of 0.6 calories per second per cubicmetre of average igneous rock. As we have to account for theescape of about 0.0014 calorie[1] per square metre of the Earth'ssurface per second (assuming the rise of temperature downwards,_i.e._ the "gradient" of temperature, to be one degree centigradein 35 metres) the downward extension of such rocks might, _primafacie_, be as much as 19 kilometres.
About this calculation we have to observe that we assume theaverage radioactivity of the materials with which we have dealtat the surface to extend uniformly all the way down, _i.e._ thatour experiments reveal the average radioactivity of a radioactivecrust. There is much to be said for this assumption. The rockswhich enter into the measurements come from all depths of thecrust. It is highly probable that the less silicious, _i.e._ themore basic, rocks, mainly come from considerable depths; the moreacid or silica-rich rocks, from higher levels in the crust. Theradioactivity determined as the mean of the values for these twoclasses of rock closely agrees with that found for intermediaterocks, or rocks containing an intermediate amount of silica.Clarke contends that this last class of material probablyrepresents the average composition of the Earth's crust so far asit has been explored by us.
[1] The calorie referred to is the quantity of heat required toheat one gram of water, _i.e._ one cubic centimetre ofwater—through one degree centigrade.
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It is therefore highly probable that the value found for the meanradioactivity of acid and basic rocks, or that found forintermediate rocks, truly represents the radioactive state of thecrust to a considerable depth. But it is easy to show that wecannot with confidence speak of the thickness of this crust asdeterminable by equating the heat outflow at the surface with theheat production of this average rock.
This appears in the failure of a radioactive layer, taken at athickness of about 19-kilometres, to account for the deep-seatedhigh temperatures which we find to be indicated by volcanicphenomena at many places on the surface. It is not hard to showthat the 19-kilometre layer would account for a temperature nohigher than about 270° >C. at its base.
It is true that this will be augmented beneath the sedimentarydeposits as we shall presently see; and that it is just inassociation with these deposits that deep-seated temperatures aremost in evidence at the surface; but still the result that themaximum temperature beneath the crust in general attains a valueno higher than 270° C. is hardly tenable. We conclude, then, thatsome other source of heat exists beneath. This may be radioactivein origin and may be easily accounted for if the radioactivematerials are more sparsely distributed at the base of the uppercrust. Or, again, the heat may be primeval or original heat,still escaping from a cooling world. For our present purpose itdoes not much matter which view
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we adopt. But we must recognise that the calculated depth of 19kilometres of crust, possessing the average radioactivity of thesurface, is excessive; for, in fact, we are compelled by thefacts to recognise that some other source of heat existsbeneath.
If the observed surface gradient of temperature persisteduniformly downwards, at some 35 kilometres beneath the surfacethere would exist temperatures (of about 1000° C.) adequate tosoften basic rocks. It is probable, however, that the gradientdiminishes downwards, and that the level at which suchtemperatures exist lies rather deeper than this. It is,doubtless, somewhat variable according to local conditions; norcan we at all approximate closely to an estimate of the depth atwhich the fusion temperatures will be reached, for, in fact, theexistence of the radioactive layer very much complicates ourestimates. In what follows we assume the depth of softening tolie at about 40 kilometres beneath the surface of the normalcrust; that is 25 miles down. It is to be observed that Prestwichand other eminent geologists, from a study of the facts ofcrust-folding, etc., have arrived at similar estimates.[1] As afurther assumption we are probably not far wrong if we assign tothe radioactive part of this crust a thickness of about 10 or 12kilometres; _i.e._ six or seven miles. This is necessarily arough approximation only; but the conclusions at which
[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._
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we shall arrive are reached in their essential features allowinga wide latitude in our choice of data. We shall speak of thispart of the crust as the normal radioactive layer.
An important fact is evolved from the mathematical investigationof the temperature conditions arising from the presence of such aradioactive layer. It is found that the greatest temperature, dueto the radioactive heat everywhere evolved in the layer—_i.e._the temperature at its base—is proportional to the square of thethickness of the layer. This fact has a direct bearing on theinfluence of radioactivity upon mountain elevation; as we shallnow find.
The normal radioactive layer of the Earth is composed of rocksextending—as we assume—approximately to a depth of 12 kilometres(7.5 miles). The temperature at the base of this layer due to theheat being continually evolved in it, is, say, t1°. Now, let ussuppose, in the trough of the geosyncline, and upon the top ofthe normal layer, a deposit of, say, 10 kilometres (6.2 miles) ofsediments is formed during a long period of continentaldenudation. What is the effect of this on the temperature at thebase of the normal layer depressed beneath this load? The totalthickness of radioactive rocks is now 22 kilometres. Accordinglywe find the new temperature t2°, by the proportion t1° : t2° ::12° : 22° That is, as 144 to 484. In fact, the temperature is morethan trebled. It is true we here assume the radioactivity of thesediments
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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 beconsiderable.
Now this remarkable increase in the temperature arises entirelyfrom the condition attending the radioactive heating; andinvolves something _additional_ to the temperature conditionsdetermined by the mere depression and thickening of the crust asin the Babbage-Herschel theory. The latter theory only involves a_shifting_ of the temperature levels (or geotherms) into thedeposited materials. The radioactive theory involves an actualrise in the temperature at any distance from the surface; so that_the level in the crust at which the rocks are softened is nearerto the surface in the geosynclines than it is elsewhere in thenormal crust_ (Pl. XV, p. 118).
In this manner the rigid part of the crust is reduced inthickness where the great sedimentary deposits have collected. Aten-kilometre layer of sediment might result in reducing theeffective thickness of the crust by 30 per cent.; afourteen-kilometre layer might reduce it by nearly 50 per cent.Even a four-kilometre deposit might reduce the effectiveresistance of the crust to compressive forces, by 10 per cent.
Such results are, of course, approximate only. They show that asthe sediments grow in depth there is a rising of the geotherm ofplasticity—whatever its true temperature may be—graduallyreducing the thickness of that part
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of the upper crust which is bearing the simultaneously increasingcompressive stresses. Below this geotherm long-continued stressresolves itself into hydrostatic pressure; above it (there is, ofcourse, no sharp line of demarcation) the crust accumulateselastic energy. The final yielding and flexure occur when theresistant cross-section has been sufficiently diminished. It isprobable that there is also some outward hydrostaitic thrust overthe area of rising temperature, which assists in determining theupward throw of the folds.
When yielding has begun in any geosyncline, and the materials arefaulted and overthrust, there results a considerably increasedthickness. As an instance, consider the piling up of sedimentsover the existing materials of the Alps, which resulted from thecompressive force acting from south to north in the progress ofAlpine upheaval. Schmidt of Basel has estimated that from 15 to20 kilometres of rock covered the materials of the Simplon as nowexposed, at the time when the orogenic forces were actively atwork folding and shearing the beds, and injecting into theirfolds the plastic gneisses coming from beneath.[1] The lateralcompression of the area of deposition of the Laramide, alreadyreferred to, resulted in a great thickening of the deposits. Manyother cases might be cited; the effect is always in some degreenecessarily produced.
[1] Schmidt, Ec. Geol. _Helvelix_, vol. ix., No. 4, p. 590
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If time be given for the heat to accumulate in the lower depthsof the crushed-up sediments, here is an additional source ofincreased temperature. The piled-up masses of the Simplon mighthave occasioned a rise due to radioactive heating of one or twohundred degrees, or even more; and if this be added to theinterior heat, a total of from 800° to 1000° might have prevailedin the rocks now exposed at the surface of the mountain. Even alesser temperature, accompanied by the intense pressureconditions, might well occasion the appearances of thermalmetamorphism described by Weinschenk, and for which, otherwise,there is difficulty in accounting.[1]