266 EVOLUTION 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, 
