DARWINISM DEFENDED. 143 



addition it is partly that of the answer to the objection to 

 be mentioned in the next sentence. This related objection 

 is that while natural selection may produce continuous 

 gradatory adaptive change or evolution it cannot produce 

 discontinuity in the series, i. e., cannot produce separated 

 distinct species. This objection receives an answer which 

 is of the nature of an admission that natural selection wholly 

 unaided really cannot differentiate species. It must call to 

 its aid some isolation or segregation factor, and as isolation 

 is certainly most commonly effected through migration and 

 geographic means, it is usually this factor of geographic 

 isolation that natural selection must be accompanied by to 

 form new species. As Plate says : "Any particular phase 

 of the struggle for existence extinguishes all those indi- 

 viduals which do not possess certain absolutely necessary 

 characteristics. By this means there is produced a common 

 type. And only when some means of isolation is added can 

 the splitting of the species into two or more forms result. 

 Natural selection can only transform a species gradually 

 and develop it in a continuous forthright line ; alone it cannot 

 produce a divergent, tree-like evolution. This results from 

 geographic, biologic, or sexual isolation, which is in most 

 cases a form of the extensive manner of working of the 

 struggle for existence. But selection can aid in the differ- 

 entiation of a species into two or more forms, as the following 

 examples show. When all average or median-sized in- 

 dividuals of a species are killed out there remain only the 

 smallest and the largest, by which we may assume that the 

 first are saved because they can most readily conceal them- 

 selves, while the latter find in their great size a sufficient 

 protection. On the ground of this difference in size perhaps 

 both forms will be inclined to keep apart from each other 

 and if to this is added a somewhat differing habit of life, 

 two races can arise which in course of time will become 

 distinct species. From a butterfly kind of very variable 



