160 THE LESSON OF EVOLUTION 



be advantageous, as also would be the power of 

 moving about in quest of this food. So that, on 

 the supposition that free inter-crossing took place, 

 the original organisms could not have separated into 

 plants and animals. Just as a river cannot branch 

 on its way down its valley until the sea throws up a 

 mud-bank at its mouth and forms a delta, so natural 

 selection can only propel a species in one direction 

 unless isolation steps in and divides it into smaller 

 groups. 



A fourth objection has been urged, that natural 

 selection cannot explain the mutual sterility be- 

 tween individuals of different species. So far as our 

 pre'sent experience goes, it seems that complete 

 fertility between individuals of different species is 

 rare, and that nearly all are absolutely infertile. No 

 matter what the characters may be which distin- 

 guish the species whether it be form or ornament, 

 or mere colour this infertility is almost always 

 present ; and of all the innumerable differences 

 which separate species, it alone is constant, or nearly 

 so. 



Darwin has shown that sterility between species is 

 not absolute, as had previously been supposed ; and 

 therefore it is not a fatal objection to the theory of 

 the development of species. But, although he did 

 this, he failed to give any satisfactory explanation 

 of the facts; and he himself did not believe that 

 infertility could have been caused directly by natural 

 selection. He suggested that the changes in struc- 

 ture were necessarily followed by physiological 

 changes; and, as neither the great structural 



