VI DIFFICULTIES AXD OBJECTIOXS 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 ^vill be more fidly 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 Ave, 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 



