Page:The New International Encyclopædia 1st ed. v. 14.djvu/324

NATURAL SELECTION. 280 NATURAL SELECTION. this fact is apparently due the exact similarity in, or coincidence of, their views. Darwin's pre- liminarj- essay, together with that of Wallace, who had also read Malthus's Essay on Population containing nearly identical selection views, was published in 1858. In the following year ap- peared Darwin's epoch-making Or'niin of Species. ilis theory was at once accepted by (anon Tris- tram. I.yell, Hooker. Bates, Iluxky. and others in Great Britain ; in the United States of America by Asa Gray and .letTries Wynian, and in Ger- many by Haeckel, while Fritz Miiller, working on the same lines in Brazil, full- accepted and ex- tended his views. That the time was ripe for the development and growth of the evoluticniary idea was proed by its speedy and general ac- ceptance by nearl}' all working naturalists and thoughtful minds. Moreover, the selection phase was easy to untlerstand by laymen, and soon the multitude accepted the new views. The N.tlkal Selection Theory Kxpl.inei). The theory of natural selection is based on the facts of variation. As to the causes of variation, Darwin does not say much in the Origin of Spe- cies. Basing his theori,- on the fact that vari- ations arc constantly and spontaneously arising, he claimed that the favorable variations have succeeded in the struggle for existence, while those unfit or unfavorable have perished. He also remarked that as long as the cimditions of life remain the same, we have reason to believe that a modification which has already been in- herited for many generations may continue to be inherited for an almost infinite nuud)er of gen- erations. We are also told that as each land had luidergone great physical changes, we might have expected to find that organic beings have varied under nature in the same way as they have un- der domestication. "And if there be any varia- bility under Nature, it would be an unaccount- able fact if natural selection did not come into play." "If, then," he asks, "animals and plants do vary, let it be ever so little or so slowly, why should we doubt that the variations or individual dilFcrences which are in any way beneficial would be preserved and accumulated through natural selection, or the survival of the fittest ? If man can by patience select variations useful to him. why. under changing and complex conditions of life, should not variations useful to Nature's living products often ari>e. and be preserved or selected? What limit can be ])ut to this power, acting during long ages and rigidly scrutinizing the whole constitution, structure, and habits of each creature — favoring the good and rejecting the bad? 1 can see no limits to this power, in slowly and beautifully adapting each form to the most complex relations of life. (iKOMETRICAI, R.VTU) OF In<REA.Si: OF ORGAN- ISMS. "A struggle for existence."' says Darwin, "inevitably follows from the high rate at which all organic Ixdngs tend to increase." There is a vast destruction of seeds, eggs, embryos, and young. Were this not the case the earth would soon become covered with the progeny of a single pair. LinnC' pointed out that an annual plant producing two seeds only — and there is no plant nearly so unproductive as this — and these each producing two in the following year, and so on. would in twenty one years produce over a million plants. The elephant is regarded as the slowest breeder of all known animals, yet a single pair would become in the course of about seven and a half centuries, if all lived to the close of the breeding age, the ancestors of nearly 10,000,000 elejjhants. The rate of increase of an animal, each pair producing ten pairs annually, and each animal living ten years, is shown in the following table, coi)ied from ilarshall's Lectures: YEAR Pairs produced Pairs alive at end ot year 1 10 no 1.210 13,310 146,il0 2 121 3 4 14 &4I 6 10 25.937.424.i;00 Over 700,ooo,ooo.ooo,uo().oou.iH)0 20 hnmense numbers of eggs are laid by certain animals, and yet there are probably no more individuals now than centuries ago, the miniber of individuals remaining as a whole stationary. The queen bee lays during her whide life a mil- lion eggs, the conger eel is estimated to deposit 1.5,000,000, the oyster from 500,000 to 10.000,000, and a very large oyster may |)roduce een (iO,- 000,000 of eggs. "Supposing," says Marsliall, "we start with one oyster and let it juoduce 10,000,000 eggs, the average American yield, and let half, or 8,000,000. be fcnuiles ami "go on in- creasing at the same rate: in the .second genera- tion we shall have sixty-four millions of millions of female oysters. In the fiftli generation — i.e. the great-great-grandchildren of our first oy.stcr — we should liave thirty-three millions of millions of millions of millicms of millions of female oys- ters. If Me add the same nuudicr of males we should have in all 66 + .33 naughts. If we esti- mate these as oyster-shells, we shoulil have a mass more than eight times the size of the world." Darwin also claimed that natural selection "acts solely by accumulating slight, successive, favorable variation," and <'an produce no great or svulden modification. It can act only by short and slow steps, hence "the canon of alura non facit .siilhnii. which every fresh addition to our knowledge tends to make truer, is on this theory intelligible." r.sE AXD Disuse. Darwin in some cases ad- mits the action of use and disuse. In both varieties and species, he says, use and disuse seem to have produced a considerable elTcct. His exani]iles are blind cave animals, the burrowing South American 'tueutueu,' which is occasionally blind, and certain moles; also the logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic duck. Instincts he regards as having been slow- ly acquired through natural selection. S<'e FiSE- Iniieritance. The (Jeolooicai, Kecord. lie then dwells on the geological record, which, although it is very imi)erfeet, yet the facts strongly, he claims, sup- port the theory of descent with modification. The extinction of species and of whole groups of species almost inevitably follows from the prin- ciple of natural selection; for old forms arc supplanted by new and improved forms. The fact, he says, that the fossil remains of each for- mation are in some degree intermediate in char- acter lietween thosi- in the strata above and l)e- low. is simply explained by their intermediate position in the chain of descent. The grand fact