PDF - Wallace Online

32 NATURAL SELECTION Hincrease the development of their own organs, and thusmodify their structure and habits has been repeatedly andeasily refuted by all writers on the subject of varieties andspecies, and it seems to have been considered that when thiswas done the whole question has been finally settled ;butthe view here developed renders such an hypothesis quiteunnecessary, by showing that similar results must be producedby the action of principles constantly at work innature. The powerful retractile talons of the falcon andthe cat tribes have not been produced or increased by thevolition of those animals ;but among the different varietieswhich occurred in the earlier and less highly organised formsof these groups, those always survived longest which had thegreatest facilities for seizing their prey. Neither did thegiraffe acquire its long neck by desiring to reach the foliageof the more lofty shrubs, and constantly stretching its neckfor the purpose, but because any varieties which occurredamong its antitypes with a longer neck than usual at oncesecured a fresh range of pasture over the same ground as theirshorter-necked companions, and on the first scarcity of food werethereby enabled to outlive them. Even the peculiar colours ofmany animals, more especially of insects, so closely resemblingthe soil or leaves or bark on which they habituallyreside, are explained on the same principle for ; though inthe course of ages varieties of many tints may have occurred,yet those races having colours best adapted to concealment from theirenemies would inevitably survive the longest.We have alsohere an acting cause to account for that balance so oftenobserved in nature, a deficiency in one set of organs alwaysbeing compensated by an increased development of someothers powerful wings accompanying weak feet, or greatvelocity making up for the absence of defensive weapons ;for it has been shown that all varieties in which an unbalanceddeficiency occurred could not long continue theirexistence. The action of this principle is exactly like thatof the centrifugal governor of the steam-engine, which checksand corrects any irregularities almost before they becomeevident ;and in like manner no unbalanced deficiency in theanimal kingdom can ever reach any conspicuous magnitude,because it would make itself felt at the very first step, by