1 66 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. 



(i) 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. 



