296 STUDIES, SCIENTIFIC AND SOCIAL 



If now we consider the population of a species with re- 

 gard to any particular character or combination of charac- 

 ters, we may divide it into three groups — a central group 

 in which the mean or average development prevails with 

 little variation, one in which the character is excessively, 

 and one in which it is little developed. These groups 

 would not be of equal extent, the central portion — that in 

 which the mean characteristics prevailed — being, in accord- 

 ance with the law of averages, much more numerous than 

 the extremes ; perhaps twice or even three times as great 

 as either of them, and forming such a series as the follow- 

 ing : — Maximum development 10, mean 30, minimum 10. 

 These figures, whatever their exact proportions, would 

 probably be pretty constant, for we have no reason to 

 believe that the mean characters, or the amount of vari- 

 ation of a species, change materially from year to year or 

 from century to century ; and we may therefore look upon 

 the central and most numerous group as presenting the 

 typical form of the species, being that which is best adapted 

 to the conditions in which it has actually to exist, while 

 the extremes, being less perfectly adapted, are continually 

 weeded out by natural selection. 



Besides the individual variation above noticed in birds 

 of the same locality, another set of variations appears in 

 birds of the same species inhabiting different localities. 

 In North America birds decrease in size as they inhabit 

 localities further south, while they become larger as we 

 go north. In mammalia, on the other hand, there is 

 generally a decrease of size both north and south from a 

 central position where the species is at a maximum. 

 Strange to say, the bill of most birds increases in length 

 towards the south, sometimes relatively, but in other 

 cases absolutely, so that the smaller southern birds some- 

 times have a bill actually longer than the northern larger 

 individuals. This peculiarity occurs in the genera Quis- 

 calus, Agelseus, Troglodytes, Seiurus, &c., and is illustrated 

 by numerous figures in Mr. Allen's work. In some cases, 

 as in the American crow, the bill is so much larger in the 

 south that the Florida birds have been recognised as a 

 distinct named variety. 



