ON VARIATION. 51 



em types — afford examples of variations 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 Cervidae (deer family) are mainly rather north- 

 ern in their distribution; that the largest species occur 

 in the colder zones, and that individuals of the same 

 species increase rapidly in size toward the north. 

 Some of the species, in fact, afford some of the most 

 striking instances of northward increase in size ; among 

 which are the Virginia deer and its several representa- 

 tives in the interior of the continent and on the Pacific 

 Slope. It is also noteworthy that the most obviously 

 distinctive characteristic of the group — the large, an- 

 nually deciduous antlers — reaches its greatest devel- 

 opment at the northward. Thus all the northern spe- 

 cies, 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 rudimentary condition — a simple, slender, sharp spike, 

 or a small and singly forked one — in the tropical spe- 

 cies; the antlers declining in size much more rapidly 

 than the general size of the animal. This is true in 

 individuals of the same species as well as of the species 

 collectively. 



"The Glires (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 decid- 

 edly to northern types. There is no more striking in- 

 stance known among mammals of variation in size 



