88 ANALYSIS OP 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 Im,pregnational Selection, and of Isolation. 



The dimensional form of impregnational selection is due to the neces- 

 sity for the coordination 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. 



Dimensional isolation 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 cornpletely 

 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 interselection. — 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 coordination of these and all 

 other sexual organs and of all organs that are necessary for the suc- 



* See Appendix II. 



