PROBLEM OF MIGRATION. 449 
been attracted by this aspect of migration, but: I cannot read 
that any one of them ever considered the formidable obstacle 
provided by the Nightjar. A simpler and sounder explanation 
is that migration is caused by nothing more nor less than a lack 
of food, but this was, to my mind, hardly a solution. It fails 
in omitting to take into consideration the question of carbon 
dioxide, and it omits entirely to show that instead of being a 
mere question of ornithology, migration is really a far-reaching 
cosmical function that is intimately connected with every single 
atom of life, plant or animal, on the face of the globe. 
A far more popular explanation of the phenomenon of migra- 
tion is that the birds were crowded out from the Arctic Regions 
by the growth of the ice-cap, and that a species of nostalgia 
carries them back to the old home each spring. The actual 
observations in support of this theory are, I submit, non-existent. 
It appears to me that in order to obtain the maximum amount 
of life on the globe annual migration must always be, and has 
always been, in operation. An inherent nostalgia (acting under 
the influence of two homes) is, of course, as essential in a 
migrant as the power of flight itself. It would be as logical to 
say that the bird returns each year to the north because it has 
wings! Really this ‘‘ Polar Origin”’ theory, which has as com- 
plement the assumption (quite unsupported by paleontological 
evidence) that the Arctic Regions are the cradle of the class 
Aves, is too palpably opposed to the present one to need more 
than the briefest mention now. 
Again I would say that this essay is merely a preliminary 
sketch of a very extensive subject. While being quite alive to 
the possibility of the existence of a fatal flaw in my reasoning, 
I can state that I have not detected it up to the present; and I 
should be intensely interested in any fact, or series of facts, that 
proves me to be in error. It is almost unnecessary to add that 
the existence of perfectly sedentary birds or fishes is no obstacle. 
Theoretically, there need be no migration of land birds in some 
Tropical islands. 
