THE DIFFERENTIATION OF SPECIES. 



21 



discover each other, and, being mutually fertile, they intermarry and begin a family, which, 

 being fertile inter se, but barren in relation to the rest of the species, is practically isolated 

 from its fellows, so that any sHght peculiarity which its members may simultaneously 

 develop, or which both parents happened to possess, is not swamped by interbreeding, but 

 goes on developing, until in process of time a new species is produced in the midst of the 

 old one. 



It is needless to state that Mr. Romanes does not present his theory in such an 

 improbable form, which most readers will regard as self-condemned and needing no 

 refutation. It seems to me, however, that the theory would break down if any one of the 

 improbabilities should be absent. To make it work we must presuppose : — 1st, the special 

 variation of the reproductive organs must occur in two individuals, otherwise the possible 

 ancestor of the new species would leave no descendants ; 2nd, it must occur at the same 

 time in both ; 3rd, it must occur at the same place ; 4th, the two individuals must be of 

 opposite sexes ; 5th, they must each of them possess some other variation, or their progeny 

 would not differ from that of the rest of the species ; and, 6th, the variation must be the 

 same in both, or appear simultaneously in the majority of their children, otherwise it would 

 be swamped by interbreeding within the physiologically isolated family. 



Is it possible to conceive that, in an island inhabited by negroes, the birth of a negro 

 with a reproductive system so modified as to render its possessor sterile with the rest of the 

 negroes, but fertile with a correspondingly modified negro of the opposite sex — even 

 supposing such an extraordinary coincidence as their simultaneous appearance to occur on 

 the island — " must necessarily " cause a new race, to say nothing of species, to take its 

 origin ? 



The theory of Mr. Romanes, ingenious as it is, and plausible as it appears to be when 

 stated as an abstract proposition, is found, when practically applied, not only to demand an 

 impossible number of coincidences, but coincidences of such a character, that, once granted, 

 the additional coincidence of fertility z;«iferse, but sterility outside the family, is almost, if 

 not quite, an unnecessary incumbrance to it. Under any circumstances it amounts to 

 nothing more than Physiological Isolation, and is as inadequate to produce Evolution as 

 Geographical Isolation is. It might perhaps be admitted as an additional form of Isolation 

 were it required. The ornithologist at any rate does not require any such aid to 

 differentiation. Amongst birds there does not seem to be a shred of evidence in favour of 

 the theory. So far as is known, no species has ever been differentiated without the aid of 

 Geographical Isolation, though Evolution may have gone on to an unknown extent ; and, so 

 far as we can judge, Geographical Isolation must always, sooner or later, be followed by 



differentiation. 



It is a remarkable circumstance that only once in the seventy-five pages of Mr. 

 Romanes's paper does the word Isolation occur, and then in a footnote. In a paper 

 relating to the origin of species it is like the play of Hamlet with the part of Hamlet 

 omitted. Evolution may go on from age to age, but without Isolation no new species can 



Improbable 

 coinci- 

 dences. 



Eeductio ad 

 absurdum. 



Eemarkable 

 absence of 

 the term 

 Isolation in 

 Eomanes's 

 paper. 



