CD is not surprised at CC’s entire rejection of his views. Agrees that there is no direct proof of unlimited variation. Says natural selection should be viewed as comparable to wave theory of light: it is probable because it groups and explains a host of facts in several fields of science.

Agrees Louis Agassiz’s review [Am. J. Sci. 2d ser. 30 (1860): 142–55] is not unfair, but Agassiz misunderstands CD. His "categories of thought" are to CD merely empty words.