POTENTIAL SEGREGATION. 167 



Some of these questions are as follows : 



(2) Points needing investigation. First. Are there many cases of 

 prepotential as well as of complete potential segregation between dif- 

 ferent forms of water animals? 



Second. Is prepotential segregation always accompanied by segre- 

 gate fecundity and segregate vigor? 



Third. If not always associated, which of the three principles 

 first appears? And what are their relations to each other? 



Fourth. When allied organisms are separated by complete environal 

 segregation, are they less liable to be separated by these three prin- 

 ciples? 



Darwin has in several places referred to the influence of prepotency 

 in pollen, and in two places I have found reference to the form of pre- 

 potency that produces segregation ; but I find no intimation that he 

 regarded this or any other form of segregation as a cause of divergent 

 evolution. The effect of prepotency in pollen from another plant in 

 preventing self-fertilization is considered in the tenth chapter of his 

 work on "Cross- and Self-fertilization in the Vegetable Kingdom," pp. 

 391-400. Some very remarkable observations concerning the pre- 

 potency of pollen from another variety than that in which the stigma 

 grows are recorded in the same chapter, but no reference is there 

 made to the effect that must be produced when the pollen of each 

 variety is prepotent on the stigma of the same variety. 



In Chapter XVI of "Variation under Domestication " it is suggested 

 that prepotency of this kind might be a cause of different varieties of 

 double hollyhocks reproducing themselves truly when growing in one 

 bed, though there was another cause to which the freedom from 

 crossing in this case has been attributed. Again, in Chapter VIII 

 of the fifth edition of "The Origin of Species," in the section on "The 

 Origin and Causes of Sterility," Darwin, while maintaining that the 

 mutual sterility of species is not due to natural selection, refers to 

 prepotency of the kind we are now considering as a quality which, 

 occurring in ever so slight a degree, would prevent deterioration of 

 character, and which would, therefore, be an advantage to a species 

 in the process of formation, and accordingly subject to accumulation 

 through natural selection. In order to construct a possible theory 

 for the introduction of sterility between allied species by means of 

 natural selection, he finds it necessary simply to add the supposition 

 that sterility is directly caused by this prepotency. He, however, for 

 several reasons, concludes that there is no such dependence of mutual 

 sterility on the process of natural selection. Concerning the pre- 

 potency he makes no reservation, and I accordingly judge that he 



