378 PHYSICAL CONDITIONS IN GENESIS OF SPECIES 



clearly apparent, the forms thus differentiated through insular in- 

 fluences not having passed beyond the A'-arietal stage; in other cases 

 they are siDCcifically ditferent from their nearest continental allies, or 

 may even have advanced far toward generic distinctness, while their 

 origin may still remain tolerably apparent. 



Plasticity, or susceptibility to the influences of physical surround- 

 ings, often differs even among quite closely allied species, as those of 

 the same family or even genus, and different species are evidently 

 affected differently by the same circumstances. Variability in color 

 may or may not accompany variability in size or in the character of 

 particular organs. Generally, however, a species which varies greatly 

 in one feature varies to a similar degree in many others. Species 

 having a wide geographical range not only commonly run into a 

 greater or less numl)er of local races, but they generally present more 

 than the average amount of strictly individual variation, as though 

 species ranging widely in space were originally more plastic than 

 those having more circumscribed hal)itats, and were thus able more 

 casilv to adapt themselves to their surroundings; they are also more 

 persistent, their fossil remains being far more frequently met with in 

 the quaternary deposits than are those of the more local and gener- 

 ally more specialized forms. 



Geographical variation, as exhibited by the niammals and birds 

 of North America, may be sunnnarized under the following heads, 

 namely, (1) variation in general size, (2) in the size of peripheral 

 parts, and {?>) in color, the latter being subdivisible into (a) variation 

 in color with latitude and (b) with longitude. As a rule, the mannnals 

 and birds of North America increase in size from the south north- 

 ward. This is true not only of the individual representatives of each 

 species, but generally the largest species of each genus and family 

 lire northern. There are, however, some strongly marked exceptions, 

 in which the increase in size is in the opposite direction, or southward. 

 There is for this an obvious explanation, as will be presently shown, 

 the increase being found to be almost invariably toward the region 

 where the type or group to which the species belongs receives its great- 

 est numerical develoj^ment and where the species attain the largest 

 size, and are also most specialized. Hence the representatives of a 

 given species increase in size toward its hypothetical center of dis- 

 tribution, which is in most cases doubtless also its original center 

 of dispersal. Consequently there is frequently a double decadence 

 in size within specific groups, and both in size and numerically in the 

 case of species when the center of development of the group to which 

 they belong is in the warm temperate or tropical regions. This may 

 be illustrated by reference to the distribution of the higher classes 

 of vertebrates in North America. Among the species occurring north 

 of Mexico there are very few that may not be supposed to have had 



