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IVERGENT EVOLUTION THROUGH SEGREGATION. SLL 
It is not however impossible that species that would otherwise be fer- 
tile inter se are thus held apart. In Broca’s work on “ Human Hybrid- 
ity”* there is a passage quoted from Prof. Serres, showing that it is 
very possible that this form of incompatibility may exist between cer- 
tain races of man. 
16. Potential segregation and pre-potential segregation —These are 
caused by more or less free distribution of the fertilizing element, 
together with the greater rapidity and power with which the sexual 
elements of the same species, race, or individual combine, as contrasted 
with the rapidity and power with which the elements of different spe- 
cies, 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 pre-potential 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 principal the fertilizing element from differ- 
ent 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 pre-potential segregation, which 
implies that the foreign as well as the home pollen is capable of produc- 
ing impregnation. Pre-potential segregation may be considered the 
initial form of potential segregation, the former passing through 
innumerable grades of intensity in the latter. We may therefore con- 
sider the principles as fundamentally one, though it will be convenient 
to retain both names. 
The importance of this principle in producing and preserving the 
diversities of the vegetable kingdom can hardly be overstated. 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 im- 
portance. 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 pre-potential as well as of potential seg- 
regation between different forms of water animals? 
(2) Is pre-potential segregation always accompanied by segregate 
fecundity and segregate vigor? 
* English translation published by the Anthropological Society of London, p. 28. 
