THUOUOTI CUMUr.ATlTE seghegation. 



241 



certain species may be due to a lack of correspondence in 

 length of the pollen-tubes and pistils. Such a lack of harmony 

 would perhaps account for difference of fertility in reciprocal 

 crosses. 



Segregate Structure does not usually arise till other forms of 

 Segregation have become so well established that difference of 

 structure does not make any essential difference in the amount of 

 intergeneration. It is not, however, impossible that species that 

 would otherwise be fertile inter se are thus held apart. In 

 Broca's work on ' Human Hybridity ' * there is a passage 

 quoted from Prof. Serres, showing that it is very possible that 

 this form of incompatibility may exist between certain races 

 of man. 



16. Potential Segregation and Prepotential Segregation. — These 

 are caused by more or less free distribution of the fertilizing 

 element together with tlie greater rapidity and power with which 

 the sexual elements of the same species, race, or individual com- 

 bine, as contrasted with the rapidity and power with which the 

 elements of different species, races, or individuals combine. 

 Potential Segregation is caused by the mutual impotence of the 

 contrasted forms, as is always the case between different orders 

 and classes ; and Prepotential Segregation is caused by the 

 superior influence of the fertilizing element from the same 

 species, race, or individual, as contrasted with that from any 

 other species, race, or individual, when both are applied to the 

 same female at the same time, or sometimes when the prepotent 

 element is applied many hours after the other. 



For the operation of this ^)rinciple the fertilizing element from 

 different males must be brought to the same female. 



When pollen fyom a contrasted genus, order, or class has no 

 more effect than inorganic dust, it seems appropriate that we 

 should call the result Potential Segregation rather than Prepo- 

 tential Segregation, which implies that the foreign as well as the 

 home pollen is capable of producing impregnation. Prepotential 

 Segregation may be considered the initial form of Potential 

 Segregation, the former pasising through innumerable grades of 

 intensity into the latter. We may, therefore, consider the 

 principles as fundamentally one, though it will be convenient to 

 retain both names. 



* English translation published by the Anthropological Society of Loudon, 

 p. 28. 



