November 24, 1905.] 



SCIENCE. 



663 



extent, many species of woodpeckers, jays, 

 the bob-whites, some of the pipilos, juncos, 

 wrens, titmice, etc. As to exploit all of 

 these cases ^puld require a good-sized vol- 

 ume, only two or three can be taken for 

 illustration in the present connection. 



In the first place, to facilitate treatment, 

 certain general well-known laws of climatic 

 or geographic variation may be recalled, 

 which are of so nearly universal application 

 that the exceptions, generally easily ex- 

 plainable, may be ignored. First, in rela- 

 tion to size. In the northern hemisphere, 

 in nearly all types of both birds and mam- 

 mals of obviously northern origin, there is 

 a gradual decrease in general size from the 

 north southward in the representatives of 

 a conspecific group, most marked, in the 

 case of birds, in non-migratory, or only 

 partly migratory, species, the most southern 

 representatives of such groups being from 

 one fifth to one third or more smaller than 

 the most northern representatives of the 

 same groups. At the same time, but less 

 generally, there is a relative increase in 

 certain peripheral parts, as the length of 

 the tail, the thickness or length of the bill 

 (according to its form) in birds, and often 

 the ear, tail and feet in mammals.^ 



- As already intimated, there are some excep- 

 tions to the rule of decrease in size southward, 

 which are covered by the following propositions, 

 first formally propounded in 1876 (Bull. Geol. and 

 Geogr. Surv. Territories, Vol. II., No. 4, July, 

 1876, p. 310) in relation primarily to mammals 

 and later {Bull. Nutt. Orn. Gluh, Vol. III., April, 

 1883, pp. 80-82) restated with more direct refer- 

 ence to birds, as follows: 



"(1) The maximum physical development of 

 the individual is attained lohere the conditions of 

 environment are most favorable to the life of the 

 species. Species being primarily limited in their 

 distribution by climatic conditions, their repre- 

 sentatives living at or near either of their re- 

 spective latitudinal boundaries are more or less 

 unfavorably afl'ected by the influences that finally 

 limit the range of the species. * * * Different 

 species being constitutionally fitted for different 

 climatic conditions, surroundings favorable to one 



Secondly, and coincidently with the de- 

 crease in size southward, is a change in 

 coloration, which may be described in gen- 

 eral terms as a restriction in area of all 

 white markings and a corresponding in- 

 crease in the area of the dark markings, 

 together with, generally speaking, an in- 

 crease in the intensity of color in markings 

 or areas of other tints than black or white, 

 as yellows, greens, browns, etc., and also 

 in iridescence, in birds of metallic tints. 

 The birds of latitudinally extended breed- 

 ing ranges in eastern North America rarely 

 present exceptions to these rules ; and many 



may be very unfavorable to others, even of the 

 same family or genus. Hence: 



"(2) The largest species of a group (genus, sub- 

 family, or family, as the case may be) are found 

 lohere the group to which 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 ex- 

 istence 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 'typical' or most generalized 

 representatives of a group are found also near its 

 center of distribution, out-lying forms being gen- 

 erally more or less ' aberrant ' or specialized. 

 Thus the Cervidse, though nearly cosmopolitan in 

 their distribution, attain their greatest develop- 

 ment, 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 re- 

 gions 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 develop- 

 ment in the northern species, the tropical forms 

 having them reduced almost to mere spikes, which 

 in some species never pass beyond a rudimentary 

 state. * * *" 



