242 REV. J. T. UULICK ON divergent evolution 
The importance of this principle in producing and preserving 
the diversities of the vegetable kingdom can hardly he over- 
stated. If pollen of every kind were equally potent on every 
stigma, what would the result be? What distinctions would 
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 V igour ? 
(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 
same 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 
