THROUGH CUMULATIVE SEGREGATION. 
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 5 * 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 the greater rapidity and power witli 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 principle the fertilizing element from 
different males must be brought to the same female. 
When pollen from 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 passing through innumerable grades of 
intensity into the latter. AVe 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 London, 
p. 28. 
