240 
BEY. J. T. GULICK ON DIVEEGENT EVOLUTION 
and are flowering at the same time, their 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 different 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 Pecundity, 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 structure. 
Darwin siiggests that the impossibility of a cross between 
