INTRODUCTION TO SOCIOLOGY 87
entiated as its mass is larger. According to the fossils dis- covered, it is certain that not only the general climate was modified, but that also local variations were produced. Animals whose remains have been found in countries now temperate, as France, Belgium, Germany, and England, could live only in tropical or, at least semi-tropical zones.
It is necessary to conclude from these prehistoric times, from the point of view of the philosophy of natural limits and social boundaries, that the temperature of the earth was modified at different times, and that each time the animal species have emigrated in harmony with these variations. Then naturally distinct vegetable and animal zones began to be drawn; in the beginning there were indeed, numerous zoological varieties, but their groupings were less distinct ; promiscuity was their dominant character. On the contrary, about the time of the Tertiary and Quaternary epochs each continent had already its peculiar species.
During the Pliocene and post-Pliocene epoch the center of Europe seems to have been very cold ; its temperature, from torrid, became glacial ; the animals of this part of the earth emigrated toward Africa, while those of the northern countries took their places, returning to the places of their origin when the climate became milder.
The Quaternary period, which precedes ours, does not seem to be separated from the latter by any distinct variation ; its fauna is allied to ours : there are developed here millions of mammalia and mollusks. Man is certainly a contemporary of this epoch, perhaps even of that preceding, at least as a tran- sitionary type.
Each place on the earth has its climate determined by a number of conditions, certain ones of which, however, can be considered as relatively general and permanent. The movement of our planet and its situation in relation to the sun are the general and constant external conditions upon which climate and temperature depend ; the predominance either of water or land is the general and constant internal cause.
The maritime climate differs from the continental climate.
