MOULT 599 
The subject of Additional Moult is thus intimately connected 
with the seasonal adornment of Birds, and as that properly be- 
longs to a branch of the great question of Natural Selection it 
could not be suitably entered upon here. The reader is accordingly 
referred to those excellent chapters in which Mr. Darwin! has 
treated the matter with his usual perspicuity, though even he has 
far from exhausted its varied points of interest. 
It remains to be remarked that though the annual Moult com- 
monly takes place so soon as the breeding-season is over, there are 
plenty of cases in which the change is delayed to a later period. 
This is so with the SwALLow, Hirundo rustica, which has long been 
known to moult in our mid-winter or even later, and it is generally 
the way with the Diurnal Birds-of-Prey. But unquestionably most 
birds accomplish the change much earlier, and before they leave 
their breeding-quarters for their winter haunts, thereby starting on 
one of their great annual journeys with all the external machinery 
of flight renewed and in the best condition for escaping its attend- 
ant perils. 
But the plumage is not the only part of the Bird’s integument 
that undergoes regularly periodical change. Many years ago 
Nilsson made known by a communication to the annual report of 
the Academy of Sciences of Stockholm on Zoology for 1828 (pp. 
104-106) that in certain species of Tetraonide (GROUSE) the CLAWS 
grow to an inordinate length in winter and are partly shed or worn 
off as spring comes on, and the fact has since received further 
attention (cf. Dresser, B. Hur. vii. p. 189, pl. 485). The seasonal 
elongation of the bill of REDPOLLS during summer, first announced 
as a conjecture by Gloger (Journ. fiir Orn. 1856, pp. 433-440), had 
been previously made known to the present writer by Wolley, who 
first observed it in 1853-4 (cf. Yarrell, Br. B. ed. 4, ii. p. 139). 
In both these cases, however, the getting rid of the extraneous 
growth is to a great extent a mechanical process, and therefore in 
some measure comparable with the shedding of the fringes of the 
feathers before mentioned. Not so does it seem to be with others, 
and a far stranger state of things was revealed by the observation, 
originally made by Mr. H. G. Palmer about 1865, according to Mr. 
Ridgway (U. S. Geol. Explor. 40th Parall. iv. p. 634), who himself 
confirmed it in countless instances, that irrespective of sex the 
White PrticaAn of North America, during the breeding-season, 
bears on the ridge of its bill a curious horny projection, flattened 
at the sides and roughly-triangular in shape, which is worn for 
about two months only, and then dropping off may be “ gathered 
by the bushel” on the nesting-grounds of the species, as recorded 
by Baird (Jbis, 1869, p. 350). Still more extraordinary was the 
1 The Descent of Man and Selection in relation to Sex, chaps. xili.-xvi. 
London: 1871. 
