ORIGIN AND SIGNIFICANCE OF SEX. 23S 



tions ; for if the organisms were interbred closely 

 enough to preserve the variation, degeneration and 

 sterility would probably result. On the other hand, 

 crossing would obliterate the new variation. It is 

 equally difficult to imagine how new characteristics 

 can arise by means of the sexual method of reproduc- 

 tion, when the weight of evidence goes to show that 

 its strongest tendency is either to obliterate pecu- 

 liar and rare characteristics, or to intensify those 

 already existing in both parents. It seems, there- 

 fore, that the sexual method of reproduction must 

 have some other reason for existence than the pro- 

 duction of new forms by the mixture of different 

 germ-plasms. We must attribute all new changes 

 in the construction of organisms to changes in the 

 forces that act upon the organism while in process 

 of construction, — arid, as I have pointed out, in 

 each generation forces are indirectly acting on the 

 construction of all generations to come after. Every 

 generation helps to form the hereditary impulse which 

 determines the developmental reaction of each suc- 

 ceeding individual to its environment. Where a 

 species continues for a great length of time in un- 

 changed conditions, the constant sameness of succes- 

 sive cycles of life makes the hereditary impulse fixed 

 and invariable, and the species becomes highly spe- 

 cialised to suit its surroundings. Eventually the lack 



