VARIATION 1007 
especially in certain American species, where part will be arrayed in 
snowy white and part are deeply coloured—blue, of some shade o1 
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 found ; 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 Red 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 novx-hollandizx 
(GosHAWK, p. 377), a species which may be properly considered 
dimorphic, and the extinct Notornis 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 
1 It had long been known that the smaller Blue Heron of America, Ardea 
(or Florida) cerulea, 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 4. (Demiegretta) rufa 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. wuerdemanni and <A. occidentalis. 
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. 
2 T 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 Perdiz 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. 
3 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) 
