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Show 126 NATURAL SELECTION. CHAP. IV. group, the later and more h.i g. hly perfected sub-gro1u ps, from branching out and seizing on many new p aces in the polity of Nature, will const~ntly tend to supplant and destroy the earlier and less Improved s~b-groups. Small and broken groups and sub-groups will finally tend to disappear. Looking to the. future, .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 extincti?n, will for a long period continue to increase. But w~ch groups will ultimately prevail, no man can predict; for we well know that many groups, fo~merly most extensively developed, have now become extmct. Lo~king still more remotely to the future, w~ may predict that, owing to the continued and steady 1ncre~se of the larger groups, a multitude of smal!er groups will be.come utterly extinct, and leave no modified descendants, ~nd consequently that of the species living at any one perwd, extremely few will transmit descendants to a remote futurity. I shall have to return to this subject in th.e chapter on Classification, but I may add ~hat on t~Is view of extremely few of the more anCient speCies having transmitted descendants, and on the view of all the descendants of the same species making a class, we can understand how it is that there exist but very few classes in each main division of the animal and vegetable kingdoms. Although extr.e~ely few of. the most ancient species may now have hv1ng and modified descendants, yet at the most remote geologic~! 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 ~ours~ of ages and under varying conditions of life, orgamc bemgs • CHAP. IV. SUMMARY. 127 var~ at al! in the several parts of their organisation, and I _think this ca_nnot be disputed ; if there be, owing to the high geometncal powers of increase of each species, at so~e age,. season, or year, a severe struggle for life, and ~his ~ertainly c~nnot be disputed ; then, considering the Infimte complexity of the relations of all organic beings to each other and to their conditions of existence causing . an infinite diversity in structure, constitutio~, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if var~ati?n.s useful to any organic being do occur, assuredly mdividuals thus characterised will have the best chance of being preserved in the struggle for life ; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection. Natural selection on the principle of q~alities being inherited at corres~onding ages, can mod1fy the egg, seed, or young, as easily as the adult. Amongst many anin1als, sexual selection will giv~ its aid to ordinary selection, by assuring to the most VIgorous and best adapted males the greatest number of offspring. Sexual selection will also give characters useful to the males alone, in their struggles with other males. Whether natural selection has really thus acted in nature, in 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 following chapters. But we already see how it entails extinction ; and how largely extinction has acted in the world's history, geology plainly declares. Natural selection, also, leads to divergence of |