310 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 
