266 E VOL UTION AND DISEASE. 



mammals dying in the Zoological Society's Gardens has 

 established the fact that rickets is a very common 

 disease. Indeed, so frequent is it among quadrumana 

 that half the monkeys and lemurs brought to this 

 country die rickety. This disease affects, besides men, 

 chimpanzees, orangs, gibbons, macaques, baboons, capu- 

 chins, squirrel and spider monkeys, and lemurs. Among 

 carnivora we find it in lions, tigers, hyaenas, bears of all 

 kinds, and in the domestic cat, the dog, fox, raccoon, 

 and seal. Among ruminants it occurs in deer, sheep, 

 and goats. In rodents, the beaver, porcupine, rabbit, 

 and coypu rat are affected by it. Among marsupials, 

 the kangaroos, phalangers, and opossums are most 

 liable. In birds it has been found in the emu, ostrich, 

 rhea, and pigeon. 



All who have studied the disease are of opinion that 

 it is due to deficiency of lime salts with the food. Mr. 

 T. D. A. Cockerell has argued, and I think on good 

 grounds, that the scalariform shells of some mollusks 

 may be regarded as arising from the same cause as 

 rickets in vertebrates. Thus rickets has an exceedingly 

 wide zoological distribution. 



Those singular productions known as cutaneous horns 

 are interesting in connection with the subject matter of 

 this chapter. In the Introduction some remarks were 

 made concerning such horns in relation to physiological 

 types, but the question was by no means exhausted. 



Man, in common with many mammals, possesses 

 glands in the skin, which secrete an unctuous material 

 known as sebum. Such glands are termed sebaceous, 

 and are more abundant in certain regions of the skin, 



