ONTOGENETIC VARIATIONS 603 



they have a far greater zygomatic breadth than any, averaging about 30 

 millimeters more in the males and 20 millimeters more in the females. 

 Again, while the external cranial measurements of the wild lions are less 

 than in park animals of equal age, the cranial capacity averages 50 cubic 

 centimeters more for the males and 40 cubic centimeters for the females, 

 as the cranial walls are actually thicker in the park-reared specimens. 

 Thus it will be seen that the ratios between skull measurements, which 

 have been used as specific criteria by paleontologists, may change ma- 

 terially during the life of a single individual as a direct result of forced 

 change of habit which may endure for but a generation. 



On studying species lists of African 4 and North American 5 mammals, 

 the following features are apparent: The number of described species 

 and subspecies of a genus seems to bear an inverse ratio to the size or, 

 more properly, to the migratory powers of the animal. Thus, in North 

 America, of the larger indigenous mammals there are : 



Bison, 2 varieties. 



mson, z varieties. 



Rangifer, 12 species, 2 subspecies, 



Ursus, 20 species, 12 subspecies. 



And of the smaller forms: 



Vulpes, 15 species, 9 subspecies. 

 Canis, 24 species, 2 subspecies. 

 Mustela, 28 species, 30 subspecies. 

 Soreat, 23 species, 40 subspecies. 

 Lepus, 15 species, 37 subspecies. 

 Sciurus, 36 species, 79 subspecies. 

 Microtus, 43 species, 38 subspecies. 

 Peromyscus, 43 species, 113 subspecies. 



As these are largely geographic variants, it would seem as though the 

 smaller forms have a much more limited sexual environment than have 

 the larger; so that small variations, evidently largely climatic, are more 

 apt to continue as distinctive characters, although probably frequently 

 ontogenetically repeated, but under hereditary control. Enforced migra- 

 tion or climatic change might well produce such alteration of color as to 

 show that many so-called subspecies — such, for instance, as those of 

 Peromyscus — are merely instances of local polymorphism. 



Detailed studies of the lists of East African forms contained in the 

 United States National Museum only serve to emphasize the same con- 

 clusion. Some of the so-called varieties, as of hunting dogs (Lt/caon ) , 



* N. Hollister : U. S. Nat. Mus., Bull. 99, pts. 1 and 2, 1918-1919. 

 5 G. S. Miller, Jr. : Ibid., Bull. 79, 1912. 



XL — Bull. Geol. Soc. Am., Vol. 33, 1921 



