ioo6 VARIA TION 



viduals from the soutli of Europe ; the peculiarities of Desert-forms 

 had been dwelt upon by Canon Tristram {suprk, 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 imfit for flight, while concomitantly the size of the bill 

 increases. The Dodo may be adduced as the extreme case of this 

 process, and it 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 Turtur rostratus peculiar to the Seychelles, but difler- 

 ing as yet in little more than its bigger bill and somewhat rounded 

 wing (nis, 1867, p. 355) from the more widely-ranging T.piduratus. 

 Other analogous cases could be advanced, the LophopsiUacus of 

 Mauritius (p. 216), the NycUcorax megacephalus of Kodriguez (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 {cf. 

 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 Dimorphism ^ (p. 149), a very few 

 cases of which have already been instanced. Many kinds of Owls of 

 diff"erent groups and of diff'erent 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. 4 1 9), 



^ This term is here used in a very wide sense, and in the cases under considera- 

 tion " Dichromatisni " would be more precise. 



