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Show 80 NATURAL SEI.JECTION. CHAr. IV. CHAPTER IV. NATURAL SELECTION. l S l t. its power compared with man's selection-its Natura e ec 10n- 't t ll aacs ower on characters of trifling im?ortance-1 s power. a a . o . apn d on b ot h sexes -Sexual SelectiOn-On the. gener,a. lity of mtcl-b tween individuals of the same species-Cucumstances fc:rosse~ ble and unfavourable to Natural Selection, namely, . avt o.u rao ssien cr isolation number of m. a· 'd 1 Sl t' n lVl ua s- ow ac 10 - In e1 cr o' ' . D' f Cha . Eictinction caused by Natural S~lectl?n- 1 vergence u " - rae t er, re1 a t ed to the diversity of mhab1tants of any small area, 1 S 1 t' th h d t naturalisation-Action of Natura e ec 10n, roug aD~ 0 1vergence of Character and Extinction., on the descend· anbt s · from a common pai.e nt- Explains the Groupmg of all orgamc e1ngs. How will the struggle for existence, discuss_ed _too? briefly in the last chapter, act in regard to variatiOn. . Can the principle of selection, which we have seen IS. so potent in the hands of man, apply in nature? I t?Ink we shall see that it can act most effectually. Let It be borne in mind in what an endless number ?f strange peculiarities our domestic productions, and, m a lesser degree, those under nature, vary ; an~ h~w s~rong t~: hereditary tendency is. Under domestwatwn, It :may truly said that the whole organisation beco:nes. In s_ome degree plastic. Let it be borne in mind how. Infinitely complex and close-fitting are the mutua~ relatw?s of all organic beings to each other and to their p~yswal conditions of life. Can it, then, be thought Improba~t' seeing that variations useful to man have undoubte y occurred, that other variations useful · in some wa~.;o each being in the great and complex battle of I ef should sometimes occur in the course of thousands o generati.o ns? If sueh d o occur, can we doubt (r emem- CrrAP. IV. NATURAL SELECTION. 81 bering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind ? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic . We shall best understand the probable course of natural f?election by taking the case of a country undergoing some physical change, for instance, of climate. The proportional numbers of its inhabitants would almost immediately undergo a change, and some species might become extinct. We may conclude, from what we have seen of the intimate and complex manner in which the inhabitants of each country are bound together, that any change in the numerical proportions of some of the inhabitants, independently of the change· of climate itself, would most seriously affect many of the others. If the country were open on its borders, new forms would certainly immigrate, and this also would seriously disturb the relations of some of the former inhabitants. L.et it be remembered how powerful the influence of a single introduced tree or mammal has been shown to be. But in the case of an island, or of a country partly surrounded by barriers, into which new and better adapted forms could not freely enter, we should then have places in the economy of nature which would assuredly be better filled up, if some of the ori-ginal inhabitants were in some manner modified · for had the area been open to immigration, these 's aine' . E3 |