88 ANALYSIS OF THE FOR PRINCIPLES. 



will not be necessary to give much spc e to them here. Of the forms 

 of impregnational selection, the last othe four mentioned above was 

 presented in my paper on Intensive t-gregation* as a factor of pro- 

 found influence in the intensification oracial groups. The term there 

 used is " fecundal intension " which ii ieated the result of the process 

 which I call "fecundal selection." 



7. The Dimensional Form oj Impregna nal Selection, and of Isolation. 



The dimensional form oj impregnate it selection is due to the neces- 

 sity for the coordination of the sexes i the intergenerating group, in 

 such a way that incompatibility of sie shall not interfere with im- 

 pregnation. Compatibility in this re >ect is maintained within the 

 intergenerating group; for if any indi dual is so far above or below 

 the average size as to render mating d icull . the chances are that the 

 descendants of that individual will be omparatively few, or perhaps 

 entirely wanting. This may be called imensional interselection. 



Dimensional isolation arises when 1< al varieties of birds and mam- 

 mals, that have become very divergi t in size, are brought to the 

 same district. An example is seen irbantams and Shanghai fowls. 

 There are also certain breeds i >f Ik >rses ad < if asses that are completely 

 prevented from crossing with certain oier breeds of the same species, 

 through incompatibility in size. Diiensional and structural isola- 

 tion are terms that convey a fairly di nite meaning, as it is evident 

 that the isolation must be brought ab< t In- the relations of members 

 of the same species to each other, ancnot through their relations to 

 the environment surrounding the spec s. There would, however, be 

 indefiniteness in the terms " dimension! "and "structural" selection; 

 for size and structure have survival alue in the relations of the 

 members of the species to the environn nt , as well as in their relations 

 to each other. I therefore prefer to ell the former of these factors 

 the dimensional form of impregnation selection and the latter the 

 structural form of impregnational section. If briefer terms are 

 desired, it will perhaps be allowable i use the forms "dimensional 

 interselection" and " structural intersection." 



8. The Structural Form oj Impregnai nal Selection, and Isolation. 



Structural interselection. — The male of many species, especially 

 among insects, are furnished with clsping organs for holding the 

 females during mating, and in some cas^ both sexes are thus equipped. 

 The structural form of impregnational election maintains the average 

 characters that are necessary for thecoordination of these and all 

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



* See Appendix II. 



