IMPREGNATIONAL SEGREGATION. 1 63 



GENERAL OBSERVATIONS ON GERMINAL AND KLORAL SEGREGATION. 



A fact of great importance in its bearing on the origin of varieties 

 should be here noted. Any variation, arising as a so-called sport, in 

 anv group of plants where either of these principles is acting strongly, 

 will be restrained from crossing, and will be preserved except in so far 

 as reversion takes place. Now, there is always a possibility that some 

 of the segregating branches of descent will not revert, and that, 

 through the special character which they possess in common, they will 

 some time secure the services of some insect that will give them the 

 benefit of cross-fertilization with each other without crossing with 

 other varieties. The power of attaining new adaptations may be 

 favored bv self-fertilization occasionallv interrupted by interbreeding 

 with individuals of another stock; for the latter is favorable as intro- 

 ducing vigor and variation, and the former as giving opportunity for 

 the accumulation of variations. 



These two methods of propagation are so far removed from those 

 found in the majority of species that it may be wise to consider any 

 transformation arising under such conditions as belonging to a 

 separate department of the process of evolution. Organisms that are 

 self-fertilized in all their generations seem to stand in nearer relation 

 to species entirely without the power of sexual propagation than to 

 species in which cross-fertilization is the usual method of propagation. 



(b) IMPREGNATIONAL SEGREGATION. 



Impregnational segregation is due to the different relations in which 

 the descendants of one original stock stand to each other in regard 

 to the possibility of their producing fertile, vigorous, and fully adapted 

 offspring when they consort together. 



In order that impregnational segregation should be established and 

 perpetuated, it is necessary, first, that variation should arise, from 

 which it results that those of one kind are capable of producing vig- 

 orous, adapted, and fertile offspring in greater numbers when breeding 

 with each other than when breeding with other kinds; second, that 

 mutually compatible forms should be so brought together as to insure 

 propagation through a series of generations. In order to secure this 

 second condition, it is necessary that, in the case of plants, there 

 should be some degree of local, germinal, or floral segregation, and, in 

 the ease of animals that pair, either pronounced local segregation or 

 partial local segregation, supplemented by social or sexual segregation. 

 The action of the different forms of impregnational segregation I call 

 negative segregation, for they rest on incompatibilities interfering with 

 mixed unions or allowing of no offspring, or of but few or inferior 

 offspring, as the result of mixed unions, and, unaided by posit ive i 



