Eiseley's argument that Darwin had borrowed from Blyth based on of a similarity in terminology has been disproven, on the grounds that Darwin used the term before he could have read Blyth, and because Darwin had clearly developed some of the focal planks of his theory by that point, observations made in rebuttal by Beddall 1972 and 1973 and Schwartz 1974 to Eiseley's claims 4 to 6 years before his literary executors reissued his earlier essays. See also Ospovat. The Eiseley view is repeated on the web at this site. Gould says something about this that is worth repeating, and I am indebted to a respondent named Seth Jackson for bringing it to my attention:
"The following kind of incident has occurred over and over again, ever since Darwin. An evolutionist, browsing through some pre-Darwinian tome in natural history, comes upon a description of natural selection. Aha, he says; I have found something important, a proof that Darwin wasn’t original. Perhaps I have even discovered a source of direct and nefarious pilfering by Darwin! In the most notorious of these claims, the great anthropologist and writer Loren Eiseley thought that he had detected such an anticipation in the writings of Edward Blyth. Eiseley laboriously worked through the evidence that Darwin had read (and used) Blyth’s work and, making a crucial etymological mistake along the way (Gould, 1987c), finally charged that Darwin may have pinched the central idea for his theory from Blyth. He published his case in a long article (Eiseley, 1959), later expanded by his executors into a posthumous volume entitled "Darwin and the Mysterious Mr. X" (1979)."
Yes, Blyth had discussed natural selection, but Eiseley didn’t realize – thus committing the usual and fateful error in this common line of argument – that all good biologists did so in the generations before Darwin. Natural selection ranked as a standard item in biological discourse – but with a crucial difference from Darwin’s version: the usual interpretation invoked natural selection as part of a larger argument for created permanency. Natural selection, in this negative formulation, acted only to preserve the type, constant and inviolate, by eliminating extreme variants and unfit individuals who threatened to degrade the essence of created form. Paley himself presents the following variant of this argument, doing so to refute (in later pages) a claim that modern species preserve the good designs winnowed from a much broader range of initial creations after natural selection had eliminated the less viable forms: "The hypothesis teaches, that every possible variety of being hath, at one time or other, found its way into existence (by what cause of in what manner is not said), and that those which were badly formed, perished" (Paley, 1803, pp. 70-71).
Darwin's theory therefore cannot be equated with the simple claim that natural selection operates. Nearly all his colleagues and predecessors accepted this postulate. Darwin, in his characteristic and radical way, grasped that this standard mechanism for preserving the type could be inverted, and then converted into the primary cause of evolutionary change. Natural selection obviously lies at the center of Darwin's theory, but we must recognize, as Darwin's second key postulate, the claim that natural selection acts as the creative force of evolutionary change. The essence of Darwinism cannot reside in the mere observation that natural selection operates - for everyone had long accepted a negative role for natural selection in eliminating the unfit and preserving the type."
Gould, S. (2002). The Structure of Evolutionary Theory. Cambridge, Massachusetts: The Belknap Press of Harvard University Press. Page 137. The Gould 1987c reference is to An Urchin in the Storm, but no page reference is given.
