Sympathetic Coloration 51 
It is part of the essence of selection, that it not only causes a part to 
vary till it has reached its highest pitch of adaptation, but that it 
maintains it at this pitch. This conserving influence of natural 
selection is of great importance, and was early recognised by Darwin; 
it follows naturally from the principle of the survival of the fittest. 
We understand from this how it is that a species which has 
become fully adapted to certain conditions of life ceases to vary, 
but remains “constant,” as long as the conditions of life for 7¢ remain 
unchanged, whether this be for thousands of years, or for whole 
geological epochs. But the most convincing proof of the power 
of the principle of selection lies in the innumerable multitude of 
phenomena which cannot be explained in any other way. To this 
category belong all structures which are only passively of advantage 
to the organism, because none of these can have arisen by the alleged 
Lamarckian principle. These have been so often discussed that 
we need do no more than indicate them here. Until quite recently 
the sympathetic coloration of animals—for instance, the whiteness 
of Arctic animals—was referred, at least in part, to the direct 
influence of external factors, but the facts can best be explained 
by referring them to the processes of selection, for then it is un- 
necessary to make the gratuitous assumption that many species are 
sensitive to the stimulus of cold and that others are not. The great 
majority of Arctic Jand-animals, mammals and birds, are white, and 
this proves that they were all able to present the variation which 
was most useful for them. The sable is brown, but it lives in trees, 
where the brown colouring protects and conceals it more effectively. 
The musk-sheep (Ovibos moschatus) is also brown, and contrasts sharply 
with the ice and snow, but it is protected from beasts of prey by its 
gregarious habit, and therefore it is of advantage to be visible from 
as great a distance as possible. That so many species have been 
able to give rise to white varieties does not depend on a special 
sensitiveness of the skin to the influence of cold, but to the fact that 
Mammals and Birds have a general tendency to vary towards white. 
Even with us, many birds—starlings, blackbirds, swallows, ete— 
occasionally produce white individuals, but the white variety does 
not persist, because it readily falls a victim to the carnivores. This 
is true of white fawns, foxes, deer, etc. The whiteness, therefore, 
arises from internal causes, and only persists when it is useful. 
A great many animals living in a green environment have become 
clothed in green, especially insects, caterpillars, and Mantidae, both 
persecuted and persecutors. 
That it is not the direct effect of the environment which calls 
forth the green colour is shown by the many kinds of caterpillar 
which rest on leaves and feed on them, but are nevertheless brown. 
4—2 
