1877.] -The Colors of Animals and Plants. 715 
peared at an early age, or in adults of one sex, and thus we may 
account for some of the most marked differences in this respect. 
With the exception of butterflies, the sexes are almost alike in 
the great majority of insects. The same is the case in mammals 
and reptiles, while the chief departure from the rule occurs in 
birds, though even here in very many cases the law of sexual 
likeness prevails. But in all cases where the.increasing develop- 
ment of color became disadvantageous to the female, it would be 
checked by natural selection, and thus produce those numerous 
instances of protective coloring in the female only which occur in 
these two groups of animals. 
There is also, I believe, a very important purpose and use of 
the varied colors of the higher animals, in the facility it affords 
for recognition by the sexes or by the young of the same species; _ 
and it is this use which probably fixes and determines the color- 
ation in many cases. When differences of size and form are very 
slight, color affords the only means of recognition at a distance 
or while in motion, and such a distinctive character must there- 
fore be of especial value to flying inse¢ts which are continually in 
motion, and encounter each other, as it were, by accident. This 
view offers us an explanation of the curious fact that among but- 
terflies the females of closely-allied species in the same locality 
sometimes differ considerably, while the males are much alike ; 
for as the males are the swiftest and the highest fliers and seek 
the females, it would evidently be advantageous for them to be 
able to recognize their true partners at some distance off. This 
peculiarity occurs with many species of Papilio, Diadema, Ado- 
lias, and Colias. In birds such marked differences of color are 
not required, owing to their higher organization and more perfect 
senses, which render recognition easy by means of a combination 
of very slight differential characters. This principle may, per- 
haps, however, account for some anomalies of coloration among 
the higher animals. Thus, Mr. Darwin, while admitting that 
the hare and the rabbit are colored protectively, remarks that the 
latter, while running to its burrow, is made conspicuous to the 
sportsman, and no doubt to all beasts of prey, by its upturned, 
white tail. But this very conspicuousness while running away 
may be useful as a signal and guide to the young, who are thus 
enabled to escape danger by following the older rabbits, directly 
and without asaiod: to the safety of the burrow ; and this may 
e the more important from the semi-nocturnal habits of the ani- 
mal. If this explanation is correct, and it certainly seems peon 
