242 EEV. J. T. OULICK ON DTYEEGEJfT EVOLUTION 



The importance of this principle in producing and preserving 

 the diversities of the vegetable kingdom can hardly be over- 

 stated. If pollen of every kind were equally potent on every 

 stigma, what would the result be ? What distinctions w ould 

 remain ? And if Potential Segregation is necessary for the 

 preservation of distinctions, is it not equally necessary for their 

 production ? Amongst water-animals that do not pair, the 

 same principle of Segregation is probably of equal importance. 

 Concerning this form of Segregation many questions of great 

 interest suggest themselves, answers to which are not found in 

 any investigations with which I am acquainted. Some of these 

 questions are as follows : — 



(1 ) Are there many cases of Prepotential as well as of Potential 

 Segregation between different forms of water-animals ? 



(2) Is Prepotential Segregation always accompanied by Segre- 

 gate Fecundity and Segregate Vigour? 



(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 

 Segregation, 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 prepotency that produces segregation ; but I 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 producing 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 prepotency 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 prepotent on the stigma of the 

 game variety. In the sixteenth chapter of ' Variation under 

 Domestication,' it is suggested that prepotency of this kind 

 might be a cause of different varieties of double hollyhock repro- 

 ducing themselves truly when growing in one bed ; though 

 there was another cause to which the freedom from crossing in 

 this case had been attributed. Again, in chapter viii. of the 



