312 



south, and confirm the law of increase in size toward the geographi- 

 cal center of the group to which they belong. 



The Frocyonidce are essentially a tropical family, in which regions are 

 found the largest species and the greatest variety of forms. The single 

 iNorth American species presents a marked increase in size southicard, as 

 will be fully shown later. 



The Glires, or Rodentia, are found throughout the greater part of the 

 world, but are represented by special groups in different regions. Being 

 strictly herbivorous, they are most numerously developed in the tem- 

 perate and warmer latitudes. The largest known species are tropical, 

 but others of large size are more or less boreal. In the northern hem^- 

 isphere, the largest species is the Beaver, which formerly ranged through- 

 out the temperate latitudes. Of the Muridce, the larger species are 

 southern, the smaller northern ; and there is a tendency (among some 

 of the species, at least) to an increase in size southward, as in some of 

 the varieties of Hesperomys leitcopus. The Arvicolince, on the other 

 hand, are subarctic and temperate in their distribution, and markedly 

 increase in size to the northward. Here, likewise, the largest species 

 of the group are met with. 



The Sciuridce are also a nearly cosmopolitan group, with different 

 genera and subfamilies specially characteristic of different regions. The 

 fSciurince are most numerously represented in the warm-temi^erate and 

 subtropical latitudes, where also occur the largest species. Yet some 

 of those of the more northern districts show a decided tendency to 

 diminution in size southward, while in others the decrease in this direc- 

 tion is less marked. The Arctomyince are temperate and subarctic, and 

 the largest species occur at the northward. Parry's Marmot is the most 

 boreal and much the largest. Franklin's Spermophile next succeeds, and 

 is one of the largest of the group. Spermophilus grammiirus (with its va- 

 rieties Beecheyi and Bouglassi), of about the same dimension, occupies the 

 elevated interior and the Pacific slope, extending, however, quite far 

 southward. The smallest of the group, S. Harrisii, S. spilosoma, and S. 

 mexicana^ have a more southern range. In all of these species, there is a 

 marked decrease in size to the southward in their respective represent- 

 atives, as there is among the species themselves. Arctomys and 

 Seiuropterus are boreal genera, with their larger species and varieties 

 occurring at the northward, and a northward increase in size in the 

 representatives of their several forms. 



The Leporidw of America are mainly restricted to the northern conii- 

 neut, their center of development as respects the number of species, 

 being the United States. Here occur also nearly all of the larger forms. 

 The Polar Hare, one of the largest, is strictly arctic; three or four others 

 of nearly equal size find their northern limit, with one exception, south 

 of the forty-ninth parallel. The most remarkable trait of the family is 

 the rather small degree of geographical variation its representatives 

 present, both as respects size and coloration. The difference in size 

 between the largest and smallest species is less than is often found lit 

 any co-ordinate group having the same number of species, and the 

 species themselves present great constancy of character. There is gen- 

 erally a slight decrease in size southward among individuals of the 

 same species, but sometimes the difference is scarcely perceptible. In 

 the most northerly but one* of the species {Lepus americanus), there is 

 apparently a very slight decrease (certainly no increase) in size north- 

 ward. 



* The material at hand is too scanty to afford grounds for any satisfactory geueral- 

 iaation respecting the Polar Hare. 



