ORIGIN OF LIFE, SEX, SPECIES, ETC. 233 



forms of life have been derived from lower," etc. It 

 accounts for the derivation by the gradual accumu- 

 lation through heredity and natural selection of 

 small differences, which arise in sexual organisms 

 from generation to generation through variation. 

 Between organisms outwardly resembling each 

 other, it recognizes difference of species by the fact 

 that by interbreeding they can either not repro- 

 duce at all, or else only reproduce sterile offspring. 

 It implies that a new species is not very different 

 outwardly from the species immediately preceding 

 it. It must be observed that in this explanation the 

 emphasis is laid on visible differences in organs and 

 structures composed of somatic cells. It leaves 

 unexplained the more mysterious fact that variety 

 after variety may arise, interbreed and reproduce 

 fertile, until there occurs a new variety which re- 

 sembles its immediate predecessors and other varie- 

 ties within the species, but cannot interbreed with 

 them and reproduce fertile offspring. In the pre- 

 ceding paragraphs it has been attempted to explain 

 this fact, and thereby supplement the Darwinian 

 theory of the origin of species at a place where its 

 insufficiency is otherwise apparent. When among 

 organisms which closely resemble each other in ap- 

 pearance, and which are derived from the same 

 species, new species is distinguished by the test of 

 incompetence to reproduce fertile offspring by inter- 

 breeding with the normal species from which they 

 are descended, then this inevitably implies fertile 



