THE MUTATION THEORY 109 



here especially that the account provided is at present unsatis- 

 factory and inconclusive. There seems, however, to be no serious 

 doubt that biennis and muricata each in their outward appearance 

 exhibit on the whole the features which their pollens respectively 

 carry, and that the features borne by their ovules are in many 

 respects distinct. 



The types are thus "hybrids" which breed true. The results 

 of intercrossing them each way are again " hybrids" which breed 

 true. It will be remembered that on former occasions de Vries 

 has formulated a general rule that species-hybrids breed true, 

 but that the cross-breds raised by interbreeding varieties do not. 

 One of these very cases was quoted 7 as an illustration of this 

 principle, viz: muricata Xbiennis. The grounds for this general 

 statement have always appeared to me insufficient, and with the 

 further knowledge which the new evidence provides we are 

 encouraged to hope that when a proper factorial analysis of the 

 types is instituted we shall find that the phenomenon of a con- 

 stant hybrid will be readily brought into line with the systems 

 of descent already worked out for such cases as that of the Stocks, 

 and others already mentioned. 



In further discussion of these facts de Vries makes a suggestion 

 which seems to me improbable. Since the egg-cells of muricata, 

 for instance, bear a certain group of features which are missing 

 on the male side, and conversely the pollen bears features absent 

 from the female side, he is inclined to regard the bad pollen grains 

 as the bearers of the missing elements of the male side and to 

 infer that there must similarly be defective ovules representing 

 the missing elements of the female side. No consideration is 

 adduced in support of this view beyond the simple fact that the 

 characters borne by male and female are dissimilar, whereas 

 it would be more in accord with preconception if the same sets 

 of combinations were represented in each as in a normal 

 Mendelian case. There is as yet no instance in which the absence 

 of any particular class of gametes has been shown with any 

 plausibility to be due to defective viability, though there are, of 

 course, cases in which certain classes of zygotes do not survive 



7 de Vries, Species and Varieties, 1905, p. 259. 



