310 



ince), the Leporidcv, the Castoridw, the ArmcoUnm among the Muridw, 

 the Saccomyidce, Geomyidce, etc. These rarely present au exception to 

 the general law of decrease in 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 Procyonidm. 



In some species (as I have elsewhere noticed), there probably exists a 

 double decadence in size, the individual reaching its maximum dimen- 

 sions 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 toward the 

 southern (in 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 : 



( L) The maximum physical development of the individual is attained where 

 the conditions of environment are most favorable to the life of the species. 

 Species being primarily limited in their distribution by climatic conditions, 

 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 

 effects of too high or too low a temperature, too little or too much humidity, 

 or their indirect effects acting upon the plants or other sources of food. 

 Hence the size of the individual generally correlates with the abundance 

 or scarcity of food. Diflerent species being constitutionally fitted for 

 different climatic conditions, surroundings favorable to one may be very 

 imfixvorable 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 ichich they severally belong 

 reaches its highest development, or where it has what may be termed its center 

 of distribution. In other words, species of a given group attain their 

 maximum size where the conditions of existence for the group in ques- 

 tion are the most favorable, just as the largest representatives of a spe- 

 cies are found where the conditions are most favorable for the existence 

 of the species. 



(3) The most " typical '' or most generalised representatives of a group are 

 found also near its center of distribution, outlying forms being generally more 

 or less ^^ aberrant" or specialized. Thus the Cervidw, though nearly cosnio- 

 politanin their distribution, attain their greatest development, both as re- 

 spects the size and the.numher of the species, in the temperate portions of 

 the northern hemisphere. The tropical species of this group are the 

 smallest of its representatives. Tliose of the temperate and cold-tem- 

 perate regions are the largest, where, too, the species are the most nu- 

 merous. Most of the species of this family also have a wide geograph- 

 ical range, and their representatives respective!}^ present great differ- 

 ences 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 at- 

 tain 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 the northward, we have 

 first, in the subarctic and cold-temperate regions, theAlcine and Saugerine 

 forms, species of the largest size, with heavy, large antlers. Next, 

 in the colder-temperate regions, come the Elaphine species, also of ver^^ 

 large size, with nearly the largest antlers of any of the Gervidw. We 



