312 CONCLUDING CHAPTER 



we did not actually know from direct evidence that 

 the origin of new races under cultivation is usually 

 sudden and complete. 



It is not necessary to repeat Darwin's demonstration 

 of the close analogy between the origin of varieties 

 under cultivation and the origin of species in Nature. 

 It is more to the purpose to point out that Mendel's 

 law has already been shown to hold good in the case 

 of many differences which have certainly not arisen 

 under cultivation, and that we have, moreover, sure 

 knowledge of the definite and spontaneous origin of 

 some natural species. 



Here we arrive at a point at which the evidence is 

 not yet by any means complete. We do not know 

 whether all or even many specific differences obey 

 Mendel's law on crossing, and a sharp limit is put to 

 our researches in this direction by the fact that so many 

 natural hybrids are sterile. Still less do we know 

 from direct evidence whether the majority of natural 

 species have arisen discontinuously, although there is 

 much circumstantial evidence which points to the con- 

 clusion that this must have been the case. 



Clearly this discontinuous method of variation is 

 likely to repay some further discussion. That such 

 mutation, or definite variation, is a phenomenon of 

 the germ-cells follows from the fact that every germ- 

 cell normally bears the complete specific character. 

 Bateson has shown that we must regard mutation as 

 consisting in the production of new kinds of gametes, 

 which differ from those normally characteristic of the 

 species. Such a change is most readily pictured by 



