"But, after all, what changes species may really undergo! How impossible will it be to distinguish and lay down a line, beyond which some of the so-called extinct species have never passed into recent ones."
Again, the following remarkable passage occurs in the postscript of a letter addressed to Sir John Herschel in 1836:—
"In regard to the origination of new species, I am very glad to find that you think it probable that it may be carried on through the intervention of intermediate causes. I left this rather to be inferred, not thinking it worth while to offend a certain class of persons by embodying in words what would only be a speculation." (In the same sense, see the letter to Whewell, March 7, 1837, volume ii., page 5):—
"In regard to this last subject [the changes from one set of animal and vegetable species to another]...you remember what Herschel said in his letter to me. If I had stated as plainly as he has done the possibility of the introduction or origination of fresh