130 Evolution and Adaptation 



of natural conditions, and if we recall the gradations that exist 

 in external conditions, I think we shall find that Darwin's 

 reply fails to give a satisfactory answer to the question. 



It is well known, and Darwin himself has commented on 

 it, that the same species often remains constant under very 

 diverse external conditions, both inorganic and organic. 

 Hence I think the explanation fails, in so far as it is 

 based on the accumulation by selection of small individual 

 variations that are supposed to give the individuals some 

 slight advantage under each set of external conditions. 

 Darwin admits that "this difficulty for a long time quite 

 confounded me. But I think it can be in large part ex- 

 plained." The first explanation that is offered is that areas 

 now continuous may not have been so in the past. This 

 may be true in places, but the great continents have had 

 continuous areas for a long time, and Darwin frankly ac- 

 knowledges that he "will pass over this way of explaining 

 the difficulty." The second attempt is based on the sup- 

 posed narrowness of the area, where two species, descended 

 from a common parent, overlap. In this region the change 

 is often very abrupt, and Darwin adds : — 



" To those who look at climate and the physical conditions 

 of life as the all-important elements of distribution, these facts 

 ought to cause surprise, as climate and height or depth grad- 

 uate away insensibly. But when we bear in mind that almost 

 every species, even in its metropolis, would increase immensely 

 in numbers, were it not for other competing species ; that nearly 

 all either prey on or serve as prey for others ; in short, that 

 each organic being is either directly or indirectly related in the 

 most important manner to other organic beings, — we see that 

 the range of the inhabitants of any country by no means ex- 

 clusively depends on insensibly changing physical conditions, 

 but in a large part on the presence of other species, on which 

 it lives, or by which it is destroyed, or with which it comes 

 into competition ; and as these species are already defined ob- 



