ME. Q. J. EOMANES ON PHYSIOLOGICAL SELECTION. 343 



Swamping Eppects op iNTERcnossiNa. 



On this subject Mr. Darwin writes, " Most animals and plants 

 keep to their proper homes, and do not needlessly wander about ; 

 we see this with migratory birds, which almost always return to 

 the same spot. Consequently, each newly-formed variety would 

 generally be at first local, as seems to be the common rule with 

 varieties in a state of nature ; so that similarly modified individuals 

 would soon exist in a small body together, and would often breed 

 together. If the new variety were successful in its battle for 

 life, it would slowly spread from a central district, competing 

 with and conquering the unchanged individuals on the margin of 

 an ever-increasing circle."* 



Now, to my mind, these considerations do not dispose of the 

 difficulty in question. In the first place, a very large assumption 

 is made when the newly -formed variety is spoken of as repre- 

 sented by " similarly modified individuals " — the assumption, 

 namely, that the same variation occurs simultaneously in a 

 number of individuals inhabiting the same area. Of course, if 

 this assumption were granted, there would be an end of the 

 present difficulty ; for if a sufficient number of individuals were 

 thus simultaneously and similarly modified, there need be no 

 longer any danger of the variety becoming swamped by inter- 

 crossing. But the force of the difficulty consists in the very fact 

 of this assumption being required to meet it. The theory of 

 natural selection, as such, furnishes no warrant for supposing 

 that the same beneficial variety should arise in a number of in- 

 dividuals simultaneously. On the contrary, the theory of natural 

 selection trusts to the chapter of accidents in the matter of 

 variation ; and in this chapter we read of no reasons why the 

 same beneficial variation should arise simultaneously in a sufficient 

 number of individual cases to prevent its being swamped by 

 intercrossing with the parent form. Or, to state the case in 

 other words, in whatever measure the assumption in question is 

 resorted to, in that measure is the theory of natural selection 

 confessed inadequate to furnish an explanation of the origin of 

 species. And to this must be added the important consideration 

 already adduced, namely, that a very large proportion, if not the 

 majority, of features which serve to distinguish species from 

 species are features presenting no utilitarian significance; and 



* ' Origin of Species/ ed. 6, pp. 72-3 et seq. 



