166 APPENDIX I—DIVERGENT EVOLUTION. 
Segregative structure does not usually arise till other forms of segre- 
gation have become so well established that difference of structure 
does not make any essential difference in the amount of intergenera- 
tion. It is not, however, impossible that species that would otherwise 
freely cross 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 men. 
16. Potential Segregation (or Segregative Potency) in its Two Forms, Complete 
Potential Segregation and Prepotential Segregation. 
(1) Nature of the Principle.—It is caused by the greater rapidity 
and efficiency with which the sexual elements of the same species, 
race, or individual combine. Complete potential segregation is 
caused by the mutual impotence of the contrasted forms, as is always 
the case between different orders and classes; and prepotential segre- 
gation 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 reach the same 
ovum at the same time, or sometimes when the prepotent element 
comes many hours after the other. That propagation may result 
compatible elements must meet. 
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 complete potential segregation rather than prepotential 
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 principle 
is 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 
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. 

* English translation, published by the Anthropological Society of London, p. 28. 
