PROBLEM OF MIGRATION. 445 
alternative is to die down at its departure, and arise again at its 
advent. The animal has two alternatives—it can follow the sun, 
or it can die down with the plant: migration and hybernation, 
in fact. 
If we could look upon the Polar Region as an area of life 
separated from the rest of the world, as a sort of ‘‘ garden 
enclosed”’ containing its just proportion of plant and animal 
life, and sharing its oxygen and carbon with all terrestrial life, 
hybernation would be quite sufficient. Plants and animals alike 
would die down each year, as is, indeed, the case with all the 
vegetation, and with the entire invertebrate fauna of the district. 
Migration would thus be unnecessary. It happens, however, 
that there is no dividing line between the Tropics and the Polar 
Regions. Theoretically, assuming that there were no migration, 
and that the balance of life were maintained by hybernation 
alone, we should require a perfectly regular grading of animal 
sleep—within the Tropics none at all, at the Poles six months, 
and every intervening spot from a single day right up to the full 
Polar limit. I confess that I cannot quite see exactly why 
hybernation should be the exception, and migration therule. Un- 
doubtedly it has something to do with the essentially locomotive 
powers of animals, and perhaps, too, with the vicissitudes of 
such continents as Greenland or the Antarctic Regions. This 
is more a question for the student of the evolution of migration, 
a subject that is quite beside the present paper.* It should suffice 
to repeat that the present terrestrial biological status is possible 
only by the existence of either a hybernating mass of animal life, 
or a migrant mass of animal life; and it happens that for some 
reason the method of migration and not that of hybernation 
predominates to-day. 
Viewed in this light, the phenomenon of bird migration, 
instead of being the esoteric study of the ornithologist, becomes 
part of a biological law of the utmost importance and magni- 
tude. When we consider this yearly ebb and flow backwards 
and forwards across the Equator of millions of tons of highly 
* One feels privileged to speculate on the effects of a lessening of the in- 
clination of the earth’s axis—this would be followed by the extinction of the 
migrants (or rather eo-migrants): the birds of to-day, the fishes or flying 
reptiles of other ages. 
