DIMENSIONAL AND STRUCTURAL SEGREGATION. 165 
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 spe- 
cies, genera, families, and orders of animals whose fertilizing elements 
are distributed in the water. When aided by this free distribution the 
combined effect is that of positive as well as negative segregation; for 
the free distribution of the fertilizing element, with the superior affinity 
of the two sexual elements that are mutually prepotent, secures the 
interbreeding of the species or variety producing the mutually prepo- 
tent elements. | 
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 when once complete 
the groups affected never coalesce. Though complete mutual ster- 
ility 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 dimensional segre- 
gation, structural segregation, potential segregation, segregate fecun- 
dity, segregate vigor, segregate adaptation, segregate freedom from 
competition, and segregate escape from enemies. 
14, Dimensional Segregation (or Segregative Size) is caused by Incompatibility 
in Size or Dimensions of the Individuals of the Different Breeds. 
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 individuals of extreme diver- 
gence in size are incapable of interbreeding. 
15. Structural Segregation (or Segregative Structure) 1s Caused by Lack of 
Correlation in the Size of Different Organs and by other Incompatibilities 
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, according as the male is 
of the one variety or of the other. 
