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- 
