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Show 310 ince\ the Leporidm^ the Castoridce, the Arvicolinas among the Muriday the Saccomyidce, OeomyidWj etc. These rarely present an exception to the general law of decrease iu size southward, though the variation is less ( in fact, occasionally almost nil) in some species than in the others. The more marked exceptions, or those in which there is an actual increase in size southward, occur in those families ihat reach their highest development with the tropics, as the Felidce and Procyonidct. In some species ( as I have elsewhere noticed), there probably exists a double decadence in size, the individual reaching its maximum dimensions where the conditions of environment are most favorable for the existence of the species, diminishing in size toward the northern ( through scarcity of food and severity of climate) as well as towa* d the southern ( iu consequence of the enervating influence of tropical or semi-tropical conditions) limit of its distribution. In a general way, the correlation of size with geographical distribution may be formulated in the following propositions: ( 1) The maximum physicaI development of the individual is attained vhere the conditions of environment are most favorable to the life of the speck*. Species being primarily limited in theirdistribution by climaticconditions, their representatives living at or near either of ther respective latitudinal boundaries are more or less unfavorably affected by the influences that finally limit the range of the species. These influences may be the direct effectsof toohigh or too low a temperature, too little or toomuch humidity, or their indirect effects acting upon the plants or other sources of food. Hence the size of the individual generally correlates with theabundauce or scarcity of food. Different species being constitutionally fitted for different climatic conditions, surroundings favorable to one may be very unfavorable to others, even of the same family or genus. Hence ( 2) The largest species of a group ( genus, subfamily, or family, as the case may be) are found where the group to which they severally belong readies its highest development, or where it has what may be termed its centtr of distribution. In other words, species of a given group attain their maximnm size where the conditions of existence for the group in question are the most favorable, just as the largest representatives of a species are found where the conditions are most favorable for the existence of the species. ( 3) The most" typical7' or most generalized representatives of a group are found also near its center of distribution, outlying forms being generally more or less " aberrant" or specialized. Th ns the Cervidee, though nearly cosmopolitan in their distribution, attain their greatest development, both as respects the size and the number of the species, in the temperate portions of the northern hemisphere. The tropical species of this group are the smallest of its representatives. Those of the temperate and cold- temperate regions are the largest, where, too, the species are the most numerous. Most of the species of this family also have a wide geographical range, and their representatives respectively present great differences in size with locality, namely, a very marked decrease in size to the southward. The possession of large, branching, deciduous antlers forms one of the marked features of the family. These appendages attain their greatest development in the northern species, the tropical forms having them reduced almost to mere spikes, which in some species never pass beyond a rudimentary state. Beginning at Ihe northward, we have first, iu the subarctic and cold- temperate regions, the Aleine and Saugeriue forms, species of the largest size, with heavy, large antlers. Next, in the colder- temperate regions, come the Elaphine species, also of very large size, with nearly the largest autlers of any of the Cervida. We |
