182 MAMMARY APPARATUS OF THE MAMMALIA 



appear to be enabled to develop further into 

 vibrissae. 



In another respect, too, these conditions in 

 the squirrel are of some theoretical interest — 

 namely, in so far as they throw a new light 

 on certain cases of hyperthely. I have 

 already pointed out that since the discovery 

 of the milk-line, the usual explanation of 

 accessory nipples or milk glands as dis- 

 membered parts of the mammfe has been 

 abandoned, and instead, all these formations 

 have been regarded as atavistic formations. 



Now we see that we are not justified in 

 generalizing so widely. For if in the squirrel 

 parts of the nipple primordia may separate and 

 shift to parts of the body comparatively dis- 

 tant from their origin, it is not impossible 

 for a similar process to happen occasionally 

 in other species. And so in some cases of 

 accessory mammae in the human being, dis- 

 tinguished by their abnormal position, it is 

 certainly simpler to accept this explanation 

 than the makeshift suppositions of the atavistic 

 interpretation. 



Returning to our special subject, on the 

 strength of the proof of the homology of 



