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.”2 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 
Colour. 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-coloured 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 resume 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 the most abundant forms. The remarkable cases offered by 
the genus Colaptes have been already mentioned (FLICKER, p. 258), and others of 
hardly less interest occur in the RotiErs (p. 793) and KALLEGE Pheasants (p. 476). 
1 The only notices of it I know are those by F. Boie (Jsis, 1834, pp. 386- 
396), and Fries (Arsberiittelse om nyare zoologiska Arbeten, 1834, pp. 38-45), the 
last being in Swedish. 
2 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 (cf. supra, pp. 344, 345). 
