Physiological Selection. 51 



And Mr. Wallace says : 



One of the greatest, or perhaps we may say the greatest, of 

 all the difficulties in the way of accepting the theory of natural 

 selection as a complete explanation of the origin of species, has 

 been the remarkable difference between varieties and species in 

 respect of fertility when crossed *. 



Now, in view of this conspicuous contrast, Darwin 

 suggested that species in a state of nature " will have 

 been exposed during long periods of time to more 

 uniform conditions than have domesticated varieties, 

 and [that] this may well make a wide difference in the 

 result." Now we have to remember that species, living 

 and extinct, are numbered by millions, and represent 

 every variety of type, constitution, and habits ; is 

 it probable, then, that this one peculiarity of the 

 reproductive system should be due, in so many cases, 

 to some merely incidental effect produced on that 

 system by uniform conditions of life? Again, ex 

 hypothesi, at the time when a variety is first forming, 

 the influence exercised by uniform conditions of life 

 (whatever in different cases this may happen to be) 

 cannot be present as regards that variety : yet this is 

 just the time when its infertility with the parent (or 

 allied) form is most likely to have arisen ; for it is 

 just then that the nascent variety would otherwise 

 have been most liable to extinction by free inter- 

 crossing even supposing that in the presence of such 

 intercrossing the variety could ever have come into 

 existence at all. 



Mr. Wallace meets the difficulty by arguing that 

 sterility between allied species may have been brought 

 about by the direct influence of natural selection. 



1 Darwinism, p. 152. 

 E 2 



