88 H. H. COLLINS 



Replacement occurred at about the sar|ie time over the whole 

 area, though that on the forelimbs and throat was slightly in 

 advance and the hip and hind limb were covered last. With 

 but few exceptions (probably not observed at the right time), 

 'hair islands' were seen particularly on the posterior part of the 

 dorsum, but they were obliterated within a few days by the 

 appearance of hair on the intervening patches. 



The differences in the appearance of the two sides of the body 

 are slight and in no case comparable with the contrast in the 

 coloration of individuals representing buff and dark extremes 

 within the species. 



6. DISCUSSION 



A study of the details of moult in living juvenal Peromyscus 

 discloses a greater regularity in the process than appears to be 

 characteristic of adult mammals in general. 



It is found that the change occurs more or less independently 

 on different parts of the body, suggesting tracts somewhat com- 

 parable to the pterylae in birds. This is most clearly seen in the. 

 method of moult on the dorsal surface of the head where growth 

 proceeds from the neck anteriorly, and from the tip of the snout 

 posteriorly to the ears. Then again, moult appears independ- 

 ently, although synchronously on opposite sides of the body. 



While the marked regularity of replacement of feathers on the 

 wings of birds may be regarded as an adaptation for the preserva- 

 tion of the power of flight, the sequence of moult on the various 

 pterylae, of the body proper, and the similar phenomenon in 

 mice as well could scarcely be interpreted in the same light. In 

 this connection, Dwight ('00) writes as follows: 



The important part that the blood supply plays in this plan ap- 

 pears to have been quite overlooked nor have I had the opportunity 

 to fully investigate it. I may say, however, that the radiation of the 

 moult from given points corresponds very closely to the distribution 

 of the superficial arteries, beginning where the main trunks come to 

 the surface and ending with their ultimate ramifications (p. 84). 



This same idea is suggested rather indirectly by Schultz ('16), 

 who regards the color markings of mammals as due to differen- 



