>i 



GEOGRAPHICAL VARIATION AMONG NORTH AMERICAN MAM- 

 MALS, ESPECIALLY IN RESPECT TO SIZE. 



BY J. A. ALLEX. 



FER2E (Suborder FISSIPEDIA). 



Having recently had an opportunity (through the kindness of Pro- 

 fessor Baird) of studying with some care the magnificent series of 

 skulls of the North American Mammalia belonging to the National 

 Museum (amounting often to eighty or a hundred specimens of a single 

 species), I have been strongly impressed with the different degrees of 

 variability exhibited by the representatives of the species and genera 

 of even the same family. The variation in size, for instance, with lati- 

 tude, in the Wolves and Foxes is surprisingly great, amounting in some 

 species (as will be shown later) to 25 per cent, of the average size of 

 the species, while in other species of the Ferae it is almost nil. Con- 

 trary to the general supposition, the variation in size among represent- 

 atives of the same species is not always a decrease with the decrease of 

 the latitude of the locality, but is in some cases exactly the reverse, in 

 some species there being a very considerable and indisputable increase 

 southward. This, for instance, is very markedly true of some species of 

 Felis and in Procyon lotor. Consequently, the very generally-received 

 impression that in North America the species of Mammalia diminish in 

 size southward, or with the decrease in the latitude (and altitude) of 

 the locality, requires modification. While such is generally the case, 

 the reverse of this too often occurs, with occasional instances also of a 

 total absence of variation in size with locality, to be considered as form- . 

 ing " the exceptions " necessary to " prove the rule". 



That there are such exceptions, both among Birds and Mammals, I 

 have been long aware, and long since noticed that where there is an 

 actual increase in size to the southward it occurs in species that belong 

 to families or genera that are mainly developed within the tropics, there 

 reaching their maximum development, both in respect to the number of 

 their specific representatives, and in respect to the size to which some of 

 the species attain. This fact seems also to have been observed by 

 others.* 



Most of the Mammals of North America belong to families, subfam- 

 ilies, or geneia which have their greatest development in the temperate 

 or colder poitions of the northern hemisphere, as the Cervidce, the 

 Canidce, the Mustelidce, the Sciuridce (especially the subfamily Arctomy- 



* I find that Mr. Robert Ridgway, some two years since, thus referred to this point. 

 In alluding to the smaller size of Mexican specimens of Catharpes mexicanns as com- 

 pared with specimens from Colorado, (C. mexicanus var. compusus) he says: "As we 

 find this peculiarity exactly paralleled in the Thryotharus[ludovicianus of the Atlantic 

 States, may not these facts point out a law to the effect that in genera and species in 

 the temperate zone the increase in size with latitude is toward the region of the highest de- 

 velopment of the group ?" Baird, Brewer, and Ridgway's Birds of North America, Vol. 

 Ill, App., p. 503, 1874. 



