1006 VARIATION 
viduals from the south of Europe; the peculiarities of Desert-forms 
had been dwelt upon by Canon Tristram (supra, p. 336, note) ; and 
the darkening tendency of a rainy climate observed by Mr. Vernon- 
Harcourt (Proc. Zool. Soc. 1851, p. 142) and Mr. Godman (supra, 
p. 338); while it had even been suggested to account for the 
similarity between British and Japanese birds in much the same 
way (Ibis, 1863, p. 189). 
Other and still more important modifications are connected with 
insulation, and we can scarcely doubt are brought about by it. 
Many instances could be cited of Birds in distant islands shewing a 
tendency to a shortening of the wing, in some to the extent of its 
becoming unfit for flight, while concomitantly the size of the bill 
increases. The Dopo may be adduced as the extreme case of this 
process, andit would seem excusable, as indicating the initiation of such 
a series of modifications as might end in something like that extinct 
form, to regard the 7'urtur rostratus peculiar to the Seychelles, but differ- 
ing as yet in little more than its bigger bill and somewhat rounded 
wing (bis, 1867, p. 355) from the more widely-ranging 7. picturatus. 
Other analogous cases could be advanced, the Lophopsittacus of 
Mauritius (p. 216), the Wycticoraa megacephalus of Rodriguez (p. 420), 
the Gallinula nesiotis of Tristan da Cunha (p. 590), and perhaps the 
Nesonetta of the Auckland Islands, which appears to be little else 
than a brevipennate form of the Anas chlorotis of New Zealand (Gf. 
Salvadori, Cat. B. Br. Mus. xxvii. p. 290). This branch of the subject 
cannot be pursued further here; but it should be obvious that it 
gives rise to problems of the greatest interest, and more light is 
likely to be thrown on the origin and cause of Variation by study- 
ing facts of this kind than by the abstract conjectures in which so 
many indulge. 
Remarkable as is the modification of colour exhibited by Desert- 
forms, or by those which inhabit rainy districts, it is comparatively 
speaking intelligible ; but so much cannot be said for the Variation 
that comes under the title of DmmorpHism! (p. 149), a very few 
cases of which have already been instanced. Many kinds of OWLS of 
different groups and of different countries, as before remarked (p. 
675), are subject to this curious kind of variability, which shews 
itself in enduing them with a plumage in which either grey or 
rufous predominates (p. 678); but strange to say a precisely similar 
Variation in colour is found to obtain among certain NIGHTJARS (p. 
642). It is impossible to suggest any way of accounting for this 
parallelism, the nocturnal habits of the majority of each group afford- 
ing the only similarity between them. An equally extraordinary 
Dimorphism is now known to occur in some of the HERONS (p. 419), 
1 This term is here used in a very wide sense, and in the cases under considera- 
tion ‘* Dichromatism ” would be more precise. 
