88 ANALYSIS OF THE FOUR PRINCIPLES. 
will not be necessary to give much space to them here. Of the forms 
of impregnational selection, the last of the four mentioned above was 
presented in my paper on Intensive Segregation* as a factor of pro- 
found influence in the intensification of racial groups. The term there 
used is ‘‘ fecundal intension ’’ which indicated the result of the process 
which I call ‘‘fecundal selection.” 
7. The Dimensional Form of Impregnational Selection, and of Isolation. 
The dimensional form of impregnational selection is due to the neces- 
sity for the codrdination of the sexes of the intergenerating group, in 
such a way that incompatibility of size shall not interfere with im- 
pregnation. Compatibility in this respect is maintained within the 
intergenerating group; for if any individual is so far above or below 
the average size as to render mating difficult, the chances are that the 
descendants of that individual will be comparatively few, or perhaps 
entirely wanting. This may be called dimensional interselection. 
Dimenstonal tsolation arises when local varieties of birds and mam- 
mals, that have become very divergent in size, are brought to the 
same district. An example is seen in bantams and Shanghai fowls. 
There are also certain breeds of horses and of asses that are completely 
prevented from crossing with certain other breeds of the same species, 
through incompatibility in size. Dimensional and structural isola- 
tion are terms that convey a fairly definite meaning, as it is evident 
that the isolation must be brought about by the relations of members 
of the same species to each other, and not through their relations to 
the environment surrounding the species. There would, however, be 
indefiniteness in the terms ‘‘ dimensional ”’ and ‘‘structural”’ selection ; 
for size and structure have survival value in the relations of the 
members of the species to the environment, as well as in their relations 
to each other. I therefore prefer to call the former of these factors 
the dimensional form of impregnational selection and the latter the 
structural form of impregnational selection. If briefer terms are 
desired, it will perhaps be allowable to use the forms ‘‘ dimensional 
interselection ’’ and ‘‘ structural interselection.”’ 
8. The Structural Form of Impregnational Selection, and Isolation. 
Structural vnterselectton.—The males of many species, especially 
among insects, are furnished with clasping organs for holding the 
females during mating, and in some cases both sexes are thus equipped. 
The structural form of impregnational selection maintains the average 
characters that are necessary for the codrdination of these and all 
other sexual organs and of all organs that are necessary for the suc- 

* See Appendix II. 
