Note 11 — Chapter II
Reading edition
Principles of Geology, vol. i. p. 285; "Pre-Historic Times," p. 411.
Mr. Croll believes that, owing to variations in the eccentricity of the earth's orbit "cold periods regularly recur every ten or fifteen thousand years; but that at much longer intervals the cold, owing to certain contingencies, is extremely severe, and lasts for a great length of time; and the last great glacial period occurred about two hundred and forty thousand years ago, and endured with slight alterations of climate for about one hundred and sixty thousand years."—Darwin's Origin of Species, p. 343.