ON VARIATION. 47 



ample material at his disposal. Following lines al- 

 ready laid down by Prof. Spencer F. Baird, Dr. Allen 

 has shown that variations in form, size, and color are 

 directly related to latitude, and that they are not pro- 

 miscuous or multifarious, but are definite and graded. 

 I make the following extract from a summary of the 

 subject published by him in TJie Radical Revieiv (New 

 Bedford, Mass.) for May, 1877: 



"Geographical variation, as exhibited by the mam- 

 mals and birds of North America, may be summarized 

 under the following heads, namely: (i) variation in 

 general size, (2) in the size of peripheral parts, and 

 (3) in color; the latter being subdivisible into {a) vari- 

 ation in color with latitude, and (/^) with longitude. 

 As a rule, the mammals and birds of North America 

 increase in size from the south northward. This is 

 true, not only of the individual representatives of each 

 species, but generally the largest species of each genus 

 and family are 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 invari- 

 ably toward the region where the type or group to 

 which the species belongs receives its greatest numer- 

 ical development, and where the species are also most 

 specialized. Hence the representatives of a given spe- 

 cies increase in size toward its hypothetical center of 

 distribution, which is in most cases doubtless also its 

 original center of dispersal. Consequently there is fre- 

 quently 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 trop- 



