186 THE THEORIES OF EVOLUTION 



double diathesis; it will be a diathetic hybrid, a Men- 

 delian hybrid. In certain cases the diatheses will exist 

 simultaneously; in other cases (those especially ob- 

 served by Mendelians) only one of them will manifest 

 itself, as happens in cases of microbian antagonism. 



The discontinuity of evolution is always due, there- 

 fore, to the presence or absence of a symbiotic microbe 

 which determines one diathesis-character or another. 

 "Mutation," Le Dantec writes, "can be accounted for 

 in the following way: A sudden morphological 

 change reveals a diathesis caused by symbiotic mi- 

 crobes introduced accidentally into the organism. 

 When those microbes exist in regions where mutating 

 plants grow, and especially when they are external 

 parasites on these plants, it is easily conceived that 

 traumatism or fecundation by a pollinic tube which 

 crossed a polluted stigma, may inoculate into the bud 

 or the ovum the new active factor." 4 Let us remark 

 that Le Dantec does not give this hypothesis as a final 

 explanation of mutation whose causes he seeks else- 

 where ; he only offers it as an illustration. 



No special interpretation is necessary, however, 

 before conclusions can be drawn. The facts of Men- 

 delian heredity are incontestable and there is no evi- 

 dence that they are produced by some symbiotic 

 element or are in any way abnormal. Mendel's laws 

 hold good in a large number of cases ; a larger number 



4 La Crise du trans for mis me, p. 212. 



