PHYSICAL CONDITIONS IN GENESIS OF SPECIES. 881 



Instances of increase in size northward among the Carnivora of 

 North America are so generally the rule that further space need not 

 be taken in recounting examples in detail. It may suffice to state that 

 the badger {Taxidea americana) , the marten {MusteJa ainerieana)^ 

 l)ie fisher {M. pennanti), the wolverine {Gulo luscvs), and the ermine 

 (Piitorins erminevs [= longicauda^ cicognanii, norehorareiisis, 

 etc.]) — all northern types — afford examples of variation in size 

 strictly parallel with that already noticed as occurring in the foxes and 

 wolves. 



To refer briefly to other groups, it may be stated that the Cervidai 

 (deer family) are mainly rather northern in their distribution; that 

 rhe largest species occur in the colder zones, and that individuals of 

 rhe same species increase rapidly in size toward the north. Some of 

 the species in fact aiford some of the most striking instances of 

 •lorthward increase in size, among which are the common Virginia 

 deer and its several representatives in the interior of the continent 

 and on the Pacific slope. It is also noteworthy that the most obvi- 

 ously distinctive characteristic t>f the group — the large, annually 

 deciduous antlers — reaches its greatest development at the north- 

 ward. Thus all the northern species, as the moose, the elk, and the 

 caribou, have branching antlers of immense size, while the antlers are 

 relatively much smaller in the species inhabiting the middle region of 

 the continent and are reduced to a rudimentar}' condition — a simple 

 slender sharp spike, or a small and singly forked one — in the tropical 

 species, the antlers declining in size much more rapidly than the 

 general size of the annnal. This is true in individuals of the same 

 species as well as of the species collectively. 



The Rodentia (the squirrels, marmots, spermophiles, mice, and 

 their affines) offer the same illustrations in respect to the law^ of 

 increase in size as the species already mentioned, the size sometimes 

 increasing to the southward, but more generally to the northward, 

 since the greater number of the species belong decidedly to northern 

 types. There is no more striking instance known among mammals 

 of variation in size with locality than that afforded by the fl.ying 

 squirrels, in which the northern race is more than one-half larger than 

 the southern; yet the two extremes are found to pass so gradually 

 the one into the other that it is hardly possible to define even a 

 southern and a northern geographical race except on the almost 

 wholly arbitrary ground of difference in size. The species, moreover. 

 is one of the most widely distributed, ranging from the Arctic regions 

 (the northern limit of forests) to Central xVmerica. 



Among birds the local differences in size are almost as strongly 

 marked as among mammals and, in the main, follow the same gen- 

 eral law. A decided increase in size southward, however, or toward 



