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Show GEOGRAPHICAL VARIATION AMONG NORTH AMERICAN MAMMALS, ESPECIALLY IN RESPECT TO SIZE. BY J. A. ALLEN. * FERxE ( Suborder FISSIPEDIA). Having recently had an opportunity ( through the kindness of Professor 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 latitude, 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 s[> eeies, while in other species of the Ferae it is almost nil. Contrary to the general supposition, the variation in size among representatives 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 Houthward. This, for instance, is very markedly true of some species of Felis and in Frocyon 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 forming " 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, subfamilies, or geneia which have their greatest development in the temperate or colder ppitions 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 mexicatws as compared with specimens from, Colorado, ( C. mexicanus var. conspusus) he sa^ s: " As we find this peculiarity exactly paralleled in the Thryotharu8[ ludovicianu8 of the Atlantio States, may not these facte point out a law to the effect that in genera and species in the temperate zone the increase in size tcith latitude is toward the region of the highest development of the group ?"- Baird, Brewer, and Ridgway's Birds of North America, Vol. Ill, App., p. 503,1874. |
