VARIATION 1005 



may be considered to have been reached) are, since in Europe 

 Gloger's attempt had failed to produce any effect/ mainly due to the 

 ornithologists of North America, and especially to Baird as before 

 said. Definite results soon followed, and in 1872 Mr, Allen in his 

 summing up was able to say truly {Proc. Bost. Soc. N. H. xv. p. 

 218), " Gradual differentiation is now known in so many cases that 

 it amounts to the demonstration of climatic variation as a general 

 law by means of which a species may be safely predicted to take on 

 a given character under certain specific climatic conditions." ^ It 

 would be impossible here to enter into particulars, suffice it to state 

 that species after species, as well as genus after genus, is proved to 

 be subject to this kind of geographical Variation, which is noticeable 

 not only in regard to the Size of the whole bird, but to the propor- 

 tion of its several parts, as Bill, Claws, Tail and Wing, as also to 

 Coloiu". Difference in the length of Wing had, it is true, been 

 noted in some species of the Old World, but the results were not 

 brought together nor their meaning made evident. As regards the 

 geographical Variation of Colour, Mr. Allen proved that in America 

 northward of Mexico it was reducible to two phases of modification, 

 a general increase of intensity toward the south and development 

 of dark markings at the expense of the light intervening spaces, so 

 that of brightly-coloiu-ed species southern individuals are the most 

 brightly coloured, and some tints, which to the northward cannot be 

 called brilliant, become vivid in a lower latitude. In respect of longi- 

 tude Variation occurs with like regularity, the differences appearing to 

 hold a direct relationship to the humidity of the climate. Thus on the 

 dry plains of the middle and western parts of the continent birds have 

 a pallid complexion, while on the Pacific slope they resimie nearly 

 the tints of the eastern form, though further to the northward, in the 

 rainy belt that extends along the coast of British Columbia, they 

 acquire a depth of colour far in excess of that which they display 

 on the Atlantic border. The value of Mr. Allen's results is very 

 much increased when we find that similar observations had long 

 before been made in regard to the Old World, only no one had been 

 at the trouble of collecting them. Thus Temminck in 1835 {Man. 

 d'Orn. iii. p. liv.) had noticed the more lively coloration of indi- 



has immeasurably tlie most abundant forms. The remarkable cases offered by 

 the genus Colaptes have been already mentioned (Flickee, p. 258), and others of 

 hardly less interest occur in the Rollers (p. 793) and Kallege Pheasants (p. 476). 



^ The only notices of it I know are those by F. Boie {Ms, 1834, pp. 386- 

 396), andj Fries {Arsberdttelse om nyare zoologiska Arbeten, 1834, pp. 38-45), the 

 last being in Swedish. 



^ He also has some remarks shewing that the usual way of accounting for such 

 variation by hybridity is untenable, though this explanation has lately been 

 revived in England by some writers, who substitute "interbreeding" for hybridity, 

 and by the shallowness of their argument prove their small capacity for reasoning 

 on the subject (c/. suprA, pp. 344, 345). 



