254 



THE AMERICAN NATURALIST [Vol. XLVIII 



The present contribution is abbreviated from a general discus- 

 sion of certain distributional problems which forms part of a 

 paper to appear from the University of California press and 

 which treats in detail of the birds and mammals of the lower 

 Colorado Valley, in California and Arizona. 



Joseph Grinnell 



YELLOW VARIETIES OF RATS 



In a recent number of the Naturalist I described a yellow 

 variety of the common rat (Mus norvegicus) which in recent 

 years made its appearance in England and is now a recognized 

 variety among fanciers. Dr. John C. Phillips and Professor L. 

 J. Cole have both called my attention to a fact which I had over- 

 looked; namely, the occurrence of a yellow variety in another 

 species of rat (Mus rattus) . Bonhote described the occurrence of 

 this variety in Egypt in 1910 and has since found by experiment 

 (1912) that the yellow variation of Mus rattus is recessive in 

 heredity precisely as it is in Mus norvegicus. The fact that the 

 yellow variation in mice is dominant in heredity, but can not be 

 obtained in a homozygous condition, stands, therefore, as a phe- 

 nomenon all the more singular and striking. 



W. E. Castle. 



BUSSEY INSTITUTION, 



March 3, 1914. 



