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 slighta 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 
