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 

 competitio.n, 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) is 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. 



