VARIATION 1007 



especially in certain American species, where part will be arrayed in 

 snowy white and part are deeply coloui'ed — blue, of some shade or 

 other, or reddish-brown as the case may be.^ In most of these 

 examples the Variation is discontinuous, for it rarely happens that 

 intermediate forms are foxmd ; and, in regard to these Herons, like 

 the Skuas before mentioned (p. 870), no question of locality has to 

 be considered, for birds of opposite colours have been observed 

 paired and breeding together. Variation indeed may be quite 

 independent of locality, as shewn by the remarkable series of speci- 

 mens of Lagopus scoticus collected almost entirely in one district by 

 Mr. Buckley (Proc. Zool. Soc. 1882, pp. 112-116). The differences 

 therein observable would almost entitle the Eed Grouse in that part, 

 at least, of Scotland to be called polymorphic, and yet in Ireland its 

 coloration seems to be monotonous (p. 391, note).^ 



In regard to Colour (p. 99) Variation of a stronger kind is 

 shewn by abnormalities which are collectively spoken of as examples 

 of Heterochrosis (p. 420), and some of them are the delight of 

 many collectors. The most common are those that tend to become 

 Albinos (p. 9), which occur in almost every group of birds, and occa- 

 sionally take permanent form, as the Australian Astur novse-hollandias 

 (GrOSHAWK, p. 377), a species Avhich may be properly considered 

 dimorphic, and the extinct Notm-nis alba (p. 592). The direct cause of 

 Albinism is easily found, and it is not much more difficult to account 

 for many cases of Melanism (p. 542),^ but that of Erythrism 

 (p. 215)* and " Xanthochroism " cannot be positively assigned 



^ It had long been known that the smaller Blue Heron of America, Ardca 

 (or Florida) cserulea, like the widely-ranging A. sacra of Polynesia, was white in 

 its young state and adopted its deep tints as it grew older ; and it began to be 

 suspected that A. (Deinifgretta)irii,fa and A. pealii might be subject to the same 

 change of colour, when in 1875 the late Dr. Brewer was able to prove that the 

 latter was but a white phase of the former, and three years later Mr. Ridgway 

 {Bull. U. S. Geol. & Geogr. Survey, iv. pp. 219-248) shewed the same relations 

 to exist between the North-American A. wu'crdemanni and A. oeddentalis. 

 Other writers have fully accepted this view, while Dr. Stejneger even suggests 

 {Stand. Nat. Hist. B. p. 7) the existence of a third phase in what has been 

 called A. wardi, so that we should here have a case of "trichromatism" which 

 would be very interesting if proved. 



^ I well remember observing at the end of two very successful days' Partridge- 

 shooting in Suffolk, in January 1859, the extraordinary amount of Variation 

 presented by the contents of the " bag " — approaching 500 in number, and nearly 

 all examples of Ferdix cinerea. I much regret that circumstances did not permit 

 my taking note of the details. At that season the birds had assumed their full 

 plumage. 



^ Melanism, it is well known, can be induced in some cage-birds by feeding them 

 with hemp-seed. Among wild birds perhaps the best known case is that of the 

 so-called Sabine's Snipe (p. 884), which is almost peculiar to the British Islands 

 and has been oftener obtained in Ireland than elsewhere. 



* American ornithologists speak of the "red" form of Owls (and Nightjars) 



