MAMMALS. 



characters. And, although it is by no means probable that 

 we have recognised more than a part of the agencies which 

 are at work, natural selection and sexual selection appear 

 to be competent to produce most of the phenomena of 

 modification observed. There remains one other influence, 

 much more obscure in its nature, to be touched upon, 

 namely, "correlation of growth" or "concomitant varia- 

 tion." When we find that when horns are developed, canine 

 teeth are absent ; or that, after a boar has been castrated, 

 his tusks cease to grow, although we may be quite unable 

 to conceive the precise manner in which the one thing in- 

 fluences the other, we can see that there is a consistency in 

 the development of the sexual weapon ceasing coincidently 

 with the destruction of the sexual apparatus, or in the fact that 

 two kinds of weapon are not developed in the same animal. 



But there are some correlations of growth of a still more 

 recondite nature, in which the connection is less obvious. 

 Of this nature is the relation which exists between pecu- 

 liarities of the skin and of the teeth : the Edentata, abnormal 

 in their skins, are different from most other Mammalia in 

 their teeth ; foetal whales, yet more aberrant in the nature 

 of their skins, have only rudimentary teeth, in the place of 

 which, after birth, plates of whalebone are found. 



Mr. Darwin ("Animals and Plants under Domestication,") 

 has collected a number of curious instances of relations exist- 

 ing between hair and teeth. In general terms it may be said 

 that any great abnormality in the hair goes hand in hand 

 with an abnormality of the teeth. Thus, there is a breed of 

 dogs found in Turkey which are almost hairless, and which 

 have very few teeth, their dentition being reduced to a single 

 molar on each side, together with a few imperfect incisors ; 

 and in the human subject inherited baldness has been found 

 to be associated with inherited deficiency of the teeth. 



But we must not go further than to say, that great 

 abnormality of hair goes hand in hand with abnormality 



T 2 



