Geographical Distribution of Indigenous Plants. 53 
the most probable is that advocated in that very valuable book on 
“Climate and Time,” by Mr. James Croll. His idea is that the high 
eccentricity of the earth’s orbit some 250,000 years ago, brought into 
play certain physical agencies which ultimately brought the glaciers 
over the country. This eccentricity of the orbit caused the warm 
currents of the Atlantic Ocean to be deflected into the Southern, in- 
stead of continuing to flow into the Northern hemisphere, thus de- 
priving the latter of at least one fifth of the entire heat it received. 
In consequence of this loss, the annual temperature was greatly 
lowered. Snow fell in the high northern regions in such quantities 
that, instead of melting, it kept accumulating year after year, and 
century after century, until a large part of the land in the Northern 
hemisphere was covered with a sheet of ice, varying from 2,000 to 
12,000 feet in thickness.* 
With such a mass of ice covering the country, all animal and 
vegetable life must either have been destroyed, or else forced to mi- 
grate southward toward warmer regions, At this period the southern 
polar seas were nearly or quite free from ice, and all that hemisphere 
was enjoying a temperate and equable climate. But after a long 
interval there came a change. The currents of the ocean were again 
deflected, this time to the north ; the same agencies piled the ice on 
the South Pole which had caused its accumulation at the North, and 
the latter in its turn enjoyed an equable and temperate climate. With 
the gradual abatement of the cold, such forms of life as had been pre- 
served in milder regions, would retire again tothe north. Some would 
reach their old station near the Arctic circle, but others would retire up 
mountains which lay in their way, and finding suitable conditions of 
climate and station, would there establish themselves. 
All around the Arctic circle before the glacial epoch of the north 
commenced, grew plants of temperate climes. The oak, the elm, the 
beech, the maple, the sycamore, the pine, and many others flourished 
in luxuriance. As far north as latitude 70°, abundant remains of 
Sequoia langsdorfit have been found. Its modern representative, 
S. sempervirens, at present growing only in limited localities in Cali- 
fornia, requires a temperature of 49° during the whole year for- its 
development ; and it must have been at least that as far north as 70° 
* Researches show that the former must have been the thickness of this sheet in Scotland, 
for unmistakable glacial markings have been found on the tops of mountains 2,000 feet 
high (Climate and Time, pp. 4389-442). And Dana in an article in Am. Jour. Sci. and Arts, 
3d series, vol. v., p. 205, et seg, gives the thickness of the New England ice sheet at 6,500 
feet, and says that on the north it ‘* was not less than 12,000 feet.” 
7 
