No. f)34] 



NOTES AND LITERATURE 



:;73 



These races do not Inn- maintain themselves in their original 

 relationships. There is somewhere within the range of this 

 young o en us. normally at or near the center, an area of optimum 

 conditions, where life is easy and there is no severe struggle for 

 existence. Here various more or less aberrant types arise and 

 are able to perpetuate themselves, spreading out in every direc- 

 tion as did the original stock, but never so far. as they are not so 



second stage a genus is in reality a well-marked species, differen- 

 tiated into many geographical races, and in the center of its 

 range being accompanied by several additional closely allied 

 species. 



numbers so that in its own little sphere the struggle for existence 

 becomes acute, and any variation from an arbitrary type is 

 unable to maintain itself. The forms occupying the limits of 

 the range of the genus as a whole (geographical or bathymet- 



than those from any other part. 



If we take the species of any genus which has reached the 



