312 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
(3) If not always associated, which of the three principles first 
appears, and what are their relations to each other? 
(4) When allied organisms are separated by complete environal seg- 
regation are they less liable to be separated by these three principles? 
Darwin has in several places referred to the influence of pre-potency 
in pollen, and in two places I have found reference to the form of pre- 
potency that produces segregation; but | find no intimation that he 
regarded this or any other form of segregation as a cause of divergent 
evolution, or aS a necessary condition for the operation of causes pro- 
ducing divergent evolution. The effect of pre-potency 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 concern- 
ing the pre-potency of pollen from another variety from 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 pre-potent on the stigma of the same variety. In the s1ix- 
teenth chapter of “ Variation under Domestication” it is suggested 
that pre-potency of this kind might be a cause of different varieties of 
double hollyhoeck reproducing themselves truly when growing in one 
bed; though there was another cause to which the freedom from cross- 
ing in this case had been attributed. Again, in chapter vill of the 
fifth edition * of the “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 pre- 
potency of the kind we are now considering as a quality which, oceur- 
ring in ever so slight a degree, would prevent deterioration of charac- 
ter, 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 imtro- 
duction of sterility between allied species by means of natural selec- 
tion, he finds it necessary simply to add the supposition that sterility 
is directly caused by this pre-potency. 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 continued to regard it as 
strengthened and developed through the action of natural selection. 
It is concerning this last point that I wish to give reasons for a dif- 
ferent opinion. I believe that qualities simply producing segregation 
van hever be accumulated by natural selection, for— 
(1) When separate generation comes in between two sections of a 
species they cease to be one aggregate, subject to modification through 
the elimination of certain parts. Both will be subject to similar forms 
of natural selection only so long as the circumstances of both and the 
 * Since ny comments on this passage were written, I have discovered that Darwin 
has omitted it from the sixth edition. 
