ZOOLOGICAL POSITION OF THE ALBINO RAT. 



269 



colonies. In other words, the common brown and our albino 

 rats cannot be distinguished from one another by their external 

 characters. 



It is nevertheless true that the albino rats which we have ex- 

 amined, are smaller in size than the brown rats in the same locali- 

 ties. In fact, the absolute size of the albino rat is nearly inter- 

 mediate between Mus rattus and Mus uorvcgicus. It is possible 



FIG. I. Copied from "Encyclopedia Britannica," in order to show the shape of 



the heads of the brown and black rats. I . Mus rattus. 



2. Jhts norvegicus. 



that the confinement in which these albinos have been reared, 

 accounts for their smaller size, as the result of lack of exercise 

 and altered conditions of life. It is possible also that we have here 

 a phenomenon similar to that described by Semper ('Si) and De 

 Varigny ('94) on snails, where the size of the animals diminished 

 with the size of the vessels in which they were reared. 



It was thought that the character of the skull misfht serve for 



<-> o 



a more exact distinction of the forms under discussion. We 

 therefore examined and compared the skulls of Mus rattns, Mus 

 norvegicus, and of the albinos. 1 



1 In order to make this comparison, it was necessary to examine as many skulls as 

 possible, and I am indebted to Professor J. A. Allen, American Museum of Natural 

 History, at New York, Professor Elliot, Field Columbian Museum at Chicago, Dr. 

 Greenman, The Wistar Institute of Anatomy at Philadelphia, and Professor Merriam, 

 National Museum at Washington, for putting at my disposal various series of skulls, 

 possessed by their several institutions. 



