MOKPHOLOGICAL CHAEACTEKS. 19 



animals, though certainly less often than in plants, peculiarities 

 of structure do occur which appear to be absolutely useless to 

 the life of the individual, although they are not rudimentary 

 organs. Such, for instance, are the colours of the skin of many, 

 and especially of marine, animals ; many expansions of the skin 

 and the sculptured character of the skin of Eleptiles, Crus- 

 taceans, and Insects would seem to be of this nature ; the rela- 

 tive positions of the various organs, which may sometimes be 

 said to be highly inappropriate, the number of the extremities 

 in various animals, and many other circumstances a^so come 

 under this head. It would be a highly important task for a 

 zoologist, and, I believe, fertile in results, to discuss all these 

 cases in detail, in order to see whether, or how far, our pre- 

 sent knowledge suffices to explain them ; i.e. to show that 

 such morphological characters do not in fact exist in animals. 

 In this place it must suffice to discuss one single example. 



It is known that the skin of Eeptiles encloses the body with 

 scales. These scales are distinguished by very various sculptur- 

 ings, highly characteristic of the different species. Ii-respective 

 of their systematic significance they appear to be of no value in 

 the life of the animal ; indeed they are viewed as ornamental, 

 without regard to the fact that they are microscopic and much 

 too delicate to be visible to other animals of their own species. 

 It might therefore seem hopeless to show the necessity for their 

 existence on Darwinian principles, and to prove that they are 

 physiologically active organs. Nevertheless recent investiga- 

 tions on this point have furnished evidence that this is possible. 



It is known that many Eeptiles, and above all the snakes, 

 cast off the whole skin at once, whereas human beings do so by 

 degrees. If by any accident they are prevented doing so, they 

 infallibly die, because the old skin has grown so tough and 

 hard that it hinders the increase in volume which is insepar- 

 able from the growth of the animal. The casting of the skin is 

 induced by the formation, on the surface of the inner epidermis, 

 of a layer of very fine and equally distributed hairs, which 

 evidently serve the purpose of mechanically raising the old 

 skin by their rigidity and position. These hairs, then, may be 

 designated as casting hairs. That they are destined and 



