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REV. J. T. GtJLICS: ON DIVERGENT EVOLUTION 



and are flowering at the same time, tlieir separate propagation is 

 preserved, in no small degree, by the Prepotential Segregation of 

 those that are most nearly allied, and by the complete Potential 

 Segregation of those that belong to diff'erent families, orders, and 

 classes. The same principle must come in to prevent the crossing 

 of different species, genera, families, and orders of animals whose 

 fertilizing elements are distributed in the water. "We must, 

 therefore, consider it a form of Positive as well as Negative 

 Segregation ; for the free distribution of the fertilizing element, 

 with the superior affinity of the two sexual elements when pro- 

 duced by those that are mutually prepotential, secures the inter- 

 breeding of those that are mutually prepotential. 



Impregnational Segregation generally exists ' between the dif- 

 ferent species of the same genus, almost always between species 

 of different genera, and always between species of different 

 families, orders, classes, and all groups of higher grade. And in 

 all these cases it is associated with other forms of segregation, 

 and whenever it has once become complete, it has never been 

 known to give way. Though complete mutual sterility never 

 gives place to complete mutual fertility, in every case where the 

 descendants of the same stock have developed into different 

 classes or orders, and in most cases where they have developed 

 into different families or genera, the reverse process has taken 

 place, and complete mutual fertility has given place to complete 

 mutual sterility. 



Under Impregnational Segregation I distinguish five principles : 

 namely. Segregate Size, Segregate Structure, Potential and Pre- 

 potential Segregation, Segregate Fecundity, and Segregate Vigour. 



14. Segregate Size is caused by incompatibility in size or 

 dimensions. 



As familiar illustrations of this form of Segregation, I may 

 mention the following : — The largest and smallest varieties of the 

 ass may run in the same pasture without any chance of crossing. 

 I have also kept Japanese bantam fowls in the same yard with 

 other breeds without any crossing. In many other species indi- 

 viduals of extreme divergence in size are incapable of inter- 

 breeding. 



15. Segregate Structure is caused by the lack of correlation in 

 the proportionate size of different organs and by other incom- 

 patibilities of structurCc 



Darwin suggests that the impossibility of a cross between 



