We have thus, though somewhat at length, gone over the evidence as to the reality and severity of the Glacial Age. It was during the continuance of such climate that Paleolithic man arrived in Europe, though it was not perhaps until its close. We must not lose sight of the fact that our principal object at present is to determine, if we can, a date for either the beginning or ending of this extraordinary season of cold, and thereby achieve an important step in determining the antiquity of man. A moment’s consideration will show us that a period of cold sufficient to produce over a large portion of the Northern Hemisphere the results we have just set forth must have a cause that is strange and far-reaching. It can not be some local cause, affecting but one continent, since the effect produced is observed as well in Europe as in America. Every year we pass through considerable changes in climate. The four seasons of the year seem to be but an annual repetition, on a very small scale of course, of the great changes in the climate of the earth that culminated in the Glacial Age; though we do not mean to say, that periods of glacial cold come and go with the regularity of our Winter. The changes in the seasons of the year are caused by the earth’s position in its orbit, and its annual revolution around the sun. It may be that the cause of the Glacial Age itself is of a similar nature; in which case it is an astronomical problem, and we ought, by calculation, to determine, with considerable accuracy, dates for the beginning and ending of this epoch.