INTRODUCTION TO SOCIOLOGY 239
to Shetland ; third, the eastern region, comprising the eastern coast of Africa, the shores of southeastern Asia, the islands of the Indies and of the Pacific Ocean ; fourth, the Arctic region, from Kamtskatka to Norway; fifth, the Antarctic zone, compris- ing Terre del Fuego, the Falkland Islands, and New Zealand.
Reptiles, crocodiles, ophidians, lizards, and even some batra- cians are the most sedentary of all the animals. They replace migration by hibernation and lethargy. Their domain par excel- lence is therefore the intertropical centers. The number of spe- cies, and of individuals of each species, diminishes in proportion as one advances toward the poles. The batracians are those which are found nearest the poles. The domain of reptiles, formerly very extended, limits itself with the differentiation of climates.
The distribution of birds is as much deprived of definite limitations as that of reptiles is circumscribed. One has yet to conjecture, since the invention of balloons and the experiments in air navigation, the enormous influence, not only economic, but moral and political, which aerial locomotion would exercise upon the social frontiers of the human species. This is a fine example of the constant correspondence between the internal and external social structure and the structure of frontiers. The powerful means of locomotion of birds naturally explains, for the greater part of them, why their geographical distribution changes only with the seasons. This influence of seasons is, however, not a simple and absolute cause. It is related to one other fundamental and primary condition the necessity of providing nourishment, of which the means are facilitated or prevented, according to seasons. These alimentary conditions, as well as variations of temperature, compel migration, as is shown in the experience of birds. "Wherever the little animals disappear that furnish the birds with nourishment; wherever the plant forms cease to grow whose grains and buds are their ordinary subsistence, there the birds disappear."
The atmospheric variations of North America being more pronounced than in our climates, the migratory species are there more numerous. Some migrate in couples, other species in
