GEOMETRICAL IlATIo or INCREAsE or ORGAN ISMS. ".\ struggle for existence," says Darwin. "inevitably follows from the high rate at which all organic !wings tend to increase." There is a vast destruction of seeds, egos, embryos, and young. Were this not the ease the earth would soon 'become covered with the progeny of a single pair. Linn6 pointed out that an annual plant producing two seed. only—and there is no plant nearly so miprialnetive as this—and these each producing two in the following year, and so on. would in twenty-one years prodnee over a million plants. The elephant is regarded as the slowest breeder of all known animals, yet a single pair would become in the eourse of about seven and a half centuries, if all lived to the close of the breeding age. the ancestors of nearly 19,000,000 elephant s.
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, copied from _Marshall's Lectures: Immense numbers of eggs are laid by certain animals. and yet there are probably no /mire individuals now than centuries ago, the number of individuals remaining as a whole stationary. The queen bee lays during her whole life a mil lion eggs. the conger eel is estimated to deposit 15,000,000, the oyster from 500.000 to 16,000,000, and a very large oyster May produce even G0, 000,000 of eggs. "Supposing," says :\larsliall, "we start with one oyster and let it produee I il,000,000 eggs. the average American yield, and let half, or 8,000,000, be females and go on in creasing at the same rate; in the second genera tion we shall have sixty-four millions of million; of female oysters. In the fifth generation—i.e. the great-great grandchildrenof our first oyster— we should have thirty-three millions of millions of millions of millions of millions of female oys ters. If we add the same omoi„,). of males we should have in all 66 -I- 33 naughts. If we esti mate these as oyster-shells, we should have a mass more than eight times the size of the world." Darwin also claimed that natural selection "acts solely by aecumulating slight, successive, favorable variation." and can produce no great or
sudden modification. It can act only by short and slow steps, hence "the canon of .V,Ittira non facit sultan,. which every fresh addition to our knowledge tends to make truer, is on this theory intelligible." ANn DisusE. Darwin in eases ad mits the action of use and disuse. Iii both varieties and species. he says, use and disuse seem to have produced a considerable effeet.
examples are blind cave animals. the burrowing South American Inentlieu,' which is oeeasionally blind. and eertain moles: also the logger-he:tiled duck. \Odell has wings incapable of flight, ill nearly the same condition as in the domestic duck. Instincts he regards is having been slow ly acquired through natural selection. See Vs:I: INHERITANCE.
Tin: (:roLoGICAL 1ZEColiD. lie then on the geological record. which. although it is very imperfeel. yet the filets strongly, he claims, sup port the theory of deseent with modifieation. The ext Met ion of speeics a nil of whole groups of species almost inevitably follows from llie prin ciple of natural selection; for old forms are supplanted by new and improved forms. The fact, he says, that llie fossil remains of each for mation are in some degree intermediate in char Heter between those in the strata above and be low, is simply explained by their intermediate position in the chain of descent. The grand fact that all extinct beings can be classed with all recent beings naturally follows from the living and the extinct being the offspring of common parents. Species have generally diverged in character during their long course of di.seent and modification, and thus the more ancient types are in some degree intermediate between existing groups. Recent forms are more iniproved and generally time specialized than the earlier ones. Vet certain forms have retrograded, While others have retained 'simple and little improved struc tures,' being what are called 'persistent types.'