No. 572] PATTERN DEVELOPMENT 



477 



way. All have a triangular patch of white at the base of 

 the throat ventrally, as a break between neck and shoulder 

 patches and a partial separation of the neck patches 

 from each other. Each has a dorsal mark of white ; in the 

 first a narrow linear break between the shoulders; in the 

 second a broader transverse mark, and in the third a 

 square patch of white occupying nearly the width of body 

 between the shoulders to the base of the neck. The white 

 throat marking increases in extent from first to third, 

 just as does the dorsal marking. Probably in time this 

 white mark, now of irregular size and appearance indi- 

 vidually, will become a permanent part of the pattern. In 

 this animal the entire head and neck are a grizzled gray 

 as far back as the posterior limit of the neck patches, and 

 the rest of the body is black. This, then, shows that the 

 pigment patches of head and neck are differentiated in 

 color as well, from the patches of the rest of the body. 

 The occurrence of white markings in the back is relati vely 

 uncommon in mammals, though white on the under sur- 

 faces is common, and, as shown by Mr. Abbott H. Thayer, 

 may be of real service to the animal as a factor in con- 

 cealment. 



In the development of white pattern-marks, the evi- 

 dence seems to show that these come in at first as small 

 and fluctuating spots, which may be of little effect in the 

 economy of the animal. Their further development might 

 lead to the extinction of the species if they render it too 

 conspicuous to enemies, unless the species at the same 

 time makes use of them or accommodates itself to their re- 

 vealing effect. Often, no doubt, they may not be a source 

 of danger at all. A case in point may be that of Sciurus 

 finlmisom, a Malayan squirrel, most of the individuals of 

 which are largely marked with white, and of which speci- 

 mens may be found side by side, varying from an almost 

 entirely pigmented condition to one of completely white 

 coat and black eyes. White squirrels are occasional in 

 other species, as albinos, but these rarely survive more 

 than a generation in the cases I have known, whereas 



