page 168 note 1 Sir Charles Lyell says (1863) geologists may freely speculate on the time when the hippopotamus “may have swum in a few summer days from rivers in the south of Spain or France to the Somme, Thames, or Severn, making timely retreat to the south before the snow and ice set in.” But, according to this theory, the natations and peregrinations of our old river-horse must have extended even further north than the Thames, as certain valley gravels at Leeds bear witness. In the discussion above I have not referred to the evidence of a mild condition of things afforded by the presence of Cyrena fluminalis and Unio littoralis. If the evidence of these shells is to be trusted, however, it would furnish an additional reason why we should reject the “migration” theory, and maintain, as the more probable hypothesis, the alternative explanation suggested by Lyell, “that when the temperature of the river water was congenial to the Cyrena above mentioned, it was also suited to the hippopotamus.” (Principles, 10th edit., vol. i., p. 194.)
