The second group of discoveries carries us back much farther, even into the earlier stages of that widespread glaciation which was the most remarkable feature of the Pleistocene period. At the periods of greatest cold "the continent of North America was deeply swathed in ice as far south as the latitude of Philadelphia, while glaciers descended into North Carolina."[4] The valleys of the Rocky Mountains also supported enormous glaciers, and a similar state of things existed at the same time in Europe. These periods of intense cold were alternated with long interglacial periods during which the climate was warmer than it is to-day. Concerning the antiquity of the Pleistocene age, which was characterized by such extraordinary vicissitudes of heat and cold, there has been, as in all questions relating to geological time, much conflict of opinion. Twenty years ago geologists often argued as if there were an unlimited fund of past time upon which to draw; but since Sir William Thomson and other physicists emphasized the point that in an antiquity very far from infinite this earth must have been a molten mass, there has been a reaction. In many instances further study has shown that less time was needed in order to effect a given change than had formerly been supposed; and so there has grown up a tendency to shorten the time assigned to geological periods. Here, as in so many other cases, the truth is doubtless to be sought within the extremes. If we adopt the magnificent argument of Dr. Croll, which seems to me still to hold its ground against all adverse criticism,[5] and regard the Glacial epoch as coincident with the last period of high eccentricity of the earth's orbit, we obtain a result that is moderate and probable. That astronomical period began about 240,000 years ago and came to an end about 80,000 years ago. During this period the eccentricity was seldom less than .04, and at one time rose to .0569. At the present time the eccentricity is .0168, and nearly 800,000 years will pass before it attains such a point as it reached during the Glacial epoch. For the last 50,000 years the departure of the earth's orbit from a circular form has been exceptionally small. [Footnote 4: _Excursions of an Evolutionist_, p. 39.] [Footnote 5: Croll, _Climate and Time in their Geological Relations_, New York, 1875; _Discussions on Climate and Cosmology_, New York, 1886; Archibald Geikie, _Text Book of Geology_, pp. 23-29, 883-909, London, 1882; James Geikie, _The Great Ice Age_, pp. 94-136, New York, 1874; _Prehistoric Europe_, pp. 558-562, London, 1881; Wallace, _Island Life_, pp. 101-225, New York, 1881. Some objections to Croll's theory may be found in Wright's _Ice Age in North America_, pp. 405-505, 585-595, New York, 1889. I have given a brief account of the theory in my _Excursions of an Evolutionist_, pp. 57-76.]