310 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
segregation, or on their being brought together by some or other form 
of positive segregation. 
When a considerable number of species of plants are commingled 
and are flowering at the same time, their separate propagation is pre- 
served, in no small degree, by the pre-potential 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 spe- 
cies, 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 produced by those that are mutually pre-potential, secures the 
inter-breeding of those that are mutually pre-potential. 
Impregnational segregation generally exists between the different 
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 onee 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 dif- 
ferent classes or orders, and in most cases where they have developed 
into different families of genera, the reverse process has taken place, 
and complete mutual fertility has given place to complete mutual ster- 
ility. 
Under impregnational segregation I distinguish five principles, 
namely, segregate size, segregate structure, potential and pre-potential 
segregation, segregate fecundity, and segregate vigor. 
14. Segregate size is caused by incompatibility in size or dimensions. 
As familiar illustrations of this form of segregation, | 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 individuals of extreme divergence in 
size are incapable of inter-breeding. 
15. Segregate structure is caused by the lack of correlation in the pro- 
portionate size of different organs and by other meompatibilities of 
structure. 
Darwin suggests that the impossibility of a cross between certain 
species may be due to a lack of correspondence in length of the pollen 
tubes and pistils. Such a lack of harmony would perhaps account for 
difference of fertility in reciprocal crosses. 
Segregate 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 inter-generation. 
