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Show 116 ' NATURAL SELECTION. [CHAP. IV. between the larger groups which areal~ trying to increase in number. One large gr~up will slowly conqu~r another large group, reduce its numbers, and thus le~se~ 1ts chance for further variation and improvement. W 1thin the same large group the later and n1ore highly perfected subgroups, frord bran.ching out and s~izing on many new places in the pohty of N atl~re, will con~tantly tend to supplant and destroy the earher and less 1mproved su?groups. Small and broken groups and sub-groups Will finally tend to disappear. Lookil!-g to. the fut~re, we can predict that the groups of organic bmngs whiCh are now large and triumphant, and which are least .bro~en up, that is, which as yet have suffered least extin~tion, will for a lonO' period continue to increase. But which groups will ulti~ately prevail, no man can predict ; for we well know that many groups, formerly most ext~nsive~y developed have now become extinct. Looking still more remdtely to the future, we may predict that, owing to the continued and steady increase of the larger groul?s, a multitude of smaller groups will become utterly extinct, and leave no modified descendants; and consequently that of the species living at any one period, extrernely few will transmit descendants to a remote futurity. I shall have to return to this subject in the chapter on Classification, but I may add that on this view of extremely few of the more ancient species having transmitted descendants, and on the view of all the descendants of the saine species making a class, we can understand ho.w it is that there exist but very few classes in each main division of the animal and vegetable kingdon1s. Although extremely few of the most ancient species may now have living and modified descendants, yet at the most remote geological period, the earth may have been as well peopled with many species of many genera, families, orders, and classes, as at the present day. Summary of Ohapter.-If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed ; if there be, owing to the SUMMARY. 117 high geometrical powers of increase of each species, at so~e age,_ season, or year, .a severe struggle for life, and ~his .certainly ca:nnot be disputed; then, considering the Infinite complexity of t~e relat_i~ns of all _organic beings to ~ach .othe: an~ to .thmr conditions of existence, causing an Infinite diversity In structure, constitution, and habits to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to ~ac~ being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any o;ganic. being do occur, assuredly individuals thus ch:=tracterised w1ll hav~ the best chance of being pres~ rved In. the ~truggle for h~e ; and from the strong prinC! Pl.e of Inheritanc~ they WI~l te~d :o produce offspring similarly charactensed. Tlu~ prin?Iple of preservation, I have called,. for the sake of brevity, Natural Selection. N a~ural selection, on :the principle of qualities being inherited at corr~sponding ages, can n1odify the egg, seed, or young, as ~asilY. as t~e a?ult: Amon~st many animals, sexual selection will give Its aid to ordinary selection by assuring to the most vigorous and best adapted males' the g:eatest number of offspring. Sexual selection will also gi:e characters useful to the males alone, in their struggles With other males. Wh~ther n~tu~~al selection has really thus acted in ~ature, I~ modifying and adapting the various forms of life to their several conditions and stations, must be judged of by the _general tenour and balance of evidence given in the_foll?wing chapters. But we already see how it entails extinction; and how largely extinction has acted in the ~orld's history, ge?logy plainly declares. Natural selectio_ n, also, leads to divergence of character; for more living b~mgs c.an be supporte~ on the same. ar~a the more they diverge In structure, habits, and constitution of which we see proof by_ looking at ~he inhabitants of a~y small spot or at. naturalised productions. Therefore during the modifica~ Ion of the descendants of any one species, and durin()' the Incessant struggle of all species to increase in number: t~e more diyersified these descendants become, the bette; Will be the1r chance of succeeding in the battle of life. |