XXV] 



DARWIN 



9 



no one of which would answer to what we mean by the word 

 * species.' No other cause but selection has yet been dis- 

 covered capable of perpetuating and giving stability to some 

 forms, and causing the disappearance of others, and therefore 

 Mr. Darwin's book, if there is any truth in it at all, has a 

 logical claim to its title. It shows how 'species,' or stable 

 forms, are produced out of unstable spontaneous variations, 

 which is certainly to trace their * origin.' The distinction of 

 ' species ' and * individual ' is equally important. A horse, or 

 a number of horses, as such, do not constitute a * species.' It 

 is the comparative permanence of the form as distinguished 

 from the ass, quagga, zebra, tapir, camel, etc., that makes 

 them one. Were there a mass of intermediate forms con- 

 necting all these animals by fine gradations, and hardly a 

 dozen individuals alike — as would probably be the case had 

 selection not acted — there might be a few horses, but there 

 would be no such thing as a species of horse. That could only 

 be produced by some power capable of eliminating interme- 

 diate forms as they arose, and preserving all of the true horse 

 type ; and such a power was first shown to exist by Mr. 

 Darwin. The origin of varieties and individuals is one thing, 

 the origin of species another." 



It is a remarkable thing that this very simple preliminary 

 misunderstanding of the very meaning of the term " species " 

 continued to appear year after year in most of the criticisms 

 of the theory of natural selection. It was put forward both 

 by mere literary critics and also by naturalists, and was in 

 many cases adduced as a discovery which completely over- 

 threw the whole of Darwin's work. So frequent was it that 

 twenty years later, when writing my ** Darwinism," I found it 

 necessary to devote the first chapter to a thorough explana- 

 tion of this point, under the heading, What are * Species,' and 

 what is meant by their ' Origin ' ? " and I think I may feel 

 confident that to those who have read that work this particular 

 purely imaginary difficulty will no longer exist. 



Soon after the "Descent of Man" appeared, I wrote to 

 Darwin, giving my impressions of the first volume, to which 

 he replied (January 30, 1871). This letter is given in the 



