THE DIFFERENTIATION OF SPECIES. 



19 



In Mr. Romanes's paper the subject is treated at considerable length. In the preamble 

 three great difficulties are set forth, which the writer regards as inexplicable by the theory 

 of Natural Selection, and an "additional theory" is propounded in order to explain them. 



The first difficulty is stated to be " the difference between natural species and 

 domesticated varieties in respect of fertility." Stated thus briefly, the difficulty is scarcely 

 apparent. It would be a matter of surprise if any two species were as fertile when crossed 

 as any two varieties when paired together, whether those varieties be natural or domesticated. 

 What Mr. Romanes really means is this: — What is the explanation of the fact that, as a 

 general rule, two forms can be, to all appearance, differentiated from each other under 

 domestication to a greater extent than has been the case in a state of nature, without 

 producing sterility when crossed? Mr. Romanes asserts that Darwin realized the force of 

 this difficulty and failed to explain it away. It may be rash to attempt anything which so 

 acute an observer and so accurate a reasoner as Darwin did not succeed in performing ; but 

 there is surely an obvious explanation of this curious fact. The animals which have been 

 domesticated by man are a few species, which, for some unknown reason, are able to adapt 

 themselves to new or strange environments with more ease than usual. That is the reason 

 why their domestication has been possible. Surely this facility to cope with the unfamiliar 

 would stand them in good stead, even in their embryonic condition. It seems reasonable 

 to expect that if the male spermatozoa be placed in a female element to which the species is 

 not accustomed, the ease with which it is able to adapt itself to a somewhat new environ- 

 ment should vary precisely in accordance with the ease with which the species is able to 

 adapt itself to new environments in later stages than that of the embryo. Amongst birds 

 fertile hybrids occur most frequently amongst the species which form the families of the 

 Phasianidte and Anatidse, the very families to which most of our domestic birds belong. 

 It may be an inexplicable fact that species differ so remarkably in their capacity to thrive 

 under the changed conditions of domestication ; but I cannot see any difficulty in the fact 

 that the embryo of a species which cannot be domesticated should perish very early, as is 

 often the case, or grow up with imperfect reproductive organs when nourished in the body 

 of an animal belonging to a different species from itself. 



Another difficulty is stated to be " that the features which serve to distinguish allied 

 species are frequently, if not usually, of a kind with which Natural Selection can have had 

 nothing whatever to do." I fully admit the truth of the statement, and I presume that 

 few naturalists would be prepared to deny that " distinctions of specific value frequently 

 have reference to structures which are without any utilitarian significance." Syllogistically 

 put, the argument stands as follows : — 



Natural Selection can only perpetuate those accidental variations which are beneficial 

 to the species ; consequently variations that arise which are not beneficial are not 

 perpetuated ; specific differences are frequently, if not usually, without any utilitarian 

 significance ; therefore they cannot have been produced by Natural Selection. To my mind 

 the false premise in this argument is the assumption that variation is accidental. Variation 



d2 



Fertility of 

 species and 

 varieties. 



Fertile 

 hybrids. 



Trivial 

 nature of 

 specific 

 differences. 



