| OCR Text |
Show 312 south, and confirm the law of increase in size toward the geographical center of the group to which they belong. The Procyanidcc are essentially a tropical family, in which regions are found the largest species and the greatest variety of forms. The single Iforth American species presents a marked increase in size southward, as will be fully shown later. The Qlires, or Bodentia, are found throughout the greater part of tbe world, but are represented by special groups in different regions. Being strictly herbivorous, they are most numerously developed in the temperate and warmer latitudes. The largest known species are tropical, but others of large size are more or less boreal. In the northern hemisphere, the largest species is the Beaver, which formerly ranged throughout the temperate latitudes. Of the Muridce, the larger species axe 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 leucopus. The Armcolince, 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. Tbe Sciurince are most numerously represented in the warm- temperate 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 direction 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 grammurus ( with its varieties Beecheyi and Dougla* si), of about the same dimension, occupies tbe elevated interior and the Pacific slope, extending, however, quite far southward. The smallest of the group, 8. Harrisii, 8. spilosomoj and & mexicana, have a more southern range. In all of these species, there is a marked decrease in size to the southward in their respective representatives, as there is among the species themselves. Arctomys ami Sciuropterus are boreal genera, with their larger species and varieties occurring at the northward, and a northward increase in size in tbe representatives of their several forms. The Leporidce of America are mainly restricted to the northern continent, their center of development as respects the number of specie^ 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, sonth 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 ID any co- ordinate group having the same number of species, and tbe species themselves present great constancy of character. There is generally a slight decrease in size southward among individuals of tbe same species, but sometimes the difference is scarcely perceptible. In the most northerly but one* of the species { Lepus americanus), therein apparently a very slight decrease ( certainly no increase) in size northward. * The material at hand is too scanty to afford grounds for any satisfactory generalization respecting the Polar Hare. |
