32 NATURAL SELECTION it 
increase the development of their own organs, and thus 
modify their structure and habits—has been repeatedly and 
easily refuted by all writers on the subject of varieties and 
species, and it seems to have been considered that when this 
was done the whole question has been finally settled ; but 
the view here developed renders such an hypothesis quite 
unnecessary, by showing that similar results must be pro- 
duced by the action of principles constantly at work in 
nature. The powerful retractile talons of the falcon and 
the cat tribes have not been produced or increased by the 
volition of those animals ; but among the different varieties 
which occurred in the earlier and less highly organised forms 
of these groups, those always survived longest which had the 
greatest facilities for seizing their prey. Neither did the 
giraffe acquire its long neck by desiring to reach the foliage 
of the more lofty shrubs, and constantly stretching its neck 
for the purpose, but because any varieties which occurred 
among its antitypes with a longer neck than usual af once 
secured a fresh range of pasture over the same ground as their 
shorter-necked companions, and on the first scarcity of food were 
thereby enabled to outlive them. Even the peculiar colours of 
many animals, more especially of insects, so closely resem- 
bling the soil or leaves or bark on which they habitually 
reside, are explained on the same principle ; for though in 
the course of ages varieties of many tints may have occurred, 
yet those races having colours best adapted to concealment from their 
enemies would imevitably survive the longest. We have also 
here an acting cause to account for that balance so often 
observed in nature,—a deficiency in one set of organs always 
being compensated by an increased development of some 
others—powerful wings accompanying weak feet, or great 
velocity making up for the absence of defensive weapons ; 
for it has been shown that all varieties in which an un- 
balanced deficiency occurred could not long continue their 
existence. The action of this principle is exactly like that 
of the centrifugal governor of the steam-engine, which checks 
and corrects any irregularities almost before they become 
evident ; and in like manner no unbalanced deficiency in the 
animal kingdom can ever reach any conspicuous magnitude, 
because it would make itself felt at the very first step, by 
