346 ANNUAL REPORT SMITHSONIAN INSTITUTION^ 1919. 
these instances are cases of incipient isolation remains to be seen. 
If this is the case, we shall get differentiation, as in the case of Cot- 
vus cornix^ the hooded crow, which has two communities, in Egypt 
and on the Persian Gulf, both of which have lost the migratory 
habit, and one of which has assumed considerable differentiation. 
It has been stated (Eagle Clark, Migration of Birds, i, pp. 15-17) 
that southern tropical regions are not suited as a nursery for the 
hardy northern birds, and if breeding were attempted in such re- 
gions the species would become extinct. 
Facts do not entirely support this view, though doubtless it is true 
as a broad principle. We have already referred to the hooded crow, 
an essentially hardy northern species and one of the few birds re- 
maining in Arctic Norway in winter, as breeding under one form 
{Corvus capellanus) on the shores of the Persian Gulf, one of the 
hottest parts of the world and eclipsing the heat of any part of 
tropical Africa, while yet another undifferentiated form is resident 
in Egypt and northern Sinai. We find a swallow {Hirundo savig- 
nii) breeding in Eg5'^pt, various forms of the white owl and kestrel 
throughout the Tropics of Asia and Africa, and other birds such as 
Saxicola torquata, the stonechat, with geographical races equally at 
home from the Arctic regions to Cape Town. 
All such distribution, as illustrated in this last paragraph, is due 
either to gradual emigration or to a regular migratory habit at some 
remote period, and has depended for its success on the initial ca- 
pacity of a species to adapt itself to new surroundings, which was 
possibly a case of necessity in the earliest attempt. 
In this connection it would be interesting to ascertain whether the 
same species, when nesting in tropical countries, lays fewer eggs in 
the clutch and rears more broods in the season than the same bird in 
more northern climes. The blackbird is said (Chapman, AVild Spain) 
to lay but three eggs in Spain, to raise three broods in Tangier 
(Favier), whilst in the Canaries the local blackbird {Turdus m. 
cdbrerae) lays very few eggs in the clutch. (Ibis, 1912, p. 597.) The 
wren {Troglodytes)^ a prolific breeder in northern climes, appears 
to lay but four eggs in the normal clutch in Sicil3^ (Ibis, 1912, p. 
171.) Is such the case among other species which have tropical rep- 
resentatives? The point is submitted to the many distinguished 
oologists whose vast collections might help to solve the problem. 
Is the normal clutch regulated by the capacity of the parents to feed 
the young (or water the young, in the case of sand grouse) , or by the 
limits of brooding surface on the parent's abdomen, or by the normal 
mortality in the species, or by what ? Even such questions have in- 
fluence on migration and distribution, for it is by no means certain 
whether birds go to the Arctic regions for reproduction, on ac- 
count of their ancient love for home, or to enable them to get suffi- 
