VI 
DIFFICULTIES AXD OBJECTIONS 
135 
poisonous fangs usually have some characteristic either of 
rattle, hood, or unusual colour, which indicates that they had 
better be left alone. 
But there is yet another form of coloration, which 
consists in special markings — bands, spots, or patches of white, 
or of bright colour, which vary in every species, and are often 
concealed when the creature is at rest but displayed when in 
motion, — as in the case of the bands and spots so frequent on 
the wings and tails of birds. Now these specific markings 
are believed, with good reason, to serve the purpose of enabling 
each species to be quickly recognised, even at a distance, by 
its fellows, especially the parents by their young and the two 
sexes by each other; and this recognition must often be an 
important factor in securing the safety of individuals, and 
therefore the wellbeing and continuance of the species. 
These interesting peculiarities will be more fully described in 
a future chapter, but they are briefly referred to here in 
order to show that the most common of all the characters by 
which species are distinguished from each other—their colours 
and markings — can be shown to be adaptive or utilitarian in 
their nature. 
But besides colour there are almost always some structural 
characters which distinguish species from species, and, as re¬ 
gards many of these also, an adaptive character can be often 
discerned. In birds, for instance, we have differences in the 
size or shape of the bill or the feet, in the length of the wing 
or the tail, and in the proportions of the several feathers of 
which these organs are composed. All these differences in 
the organs on which the very existence of birds depends, 
which determine the character of flight, facility for running 
or climbing, for inhabiting chiefly the ground or trees, and 
the kind of food that can be most easily obtained for 
themselves and their offspring, must surely be in the highest 
degree utilitarian ; although in each individual case we, in our 
ignorance of the minutiae of their life-history, may be quite 
unable to see the use. In mammalia specific differences other 
than colour usually consist in the length or shape of the ears 
and tail, in the proportions of the limbs, or in the length 
and quality of the hair on different parts of the body. As 
regards the ears and tail, one of the objections by Professor 
