OTHER THEORIES OF SPECIES-FORMING. 335 



er the development of new 

 forms. 



5. The chief requisite for evo- 

 lution is, therefore, the struggle 

 for existence and the selection 

 which results from it. 



more energetically can evolution 

 go on. New forms do not arise 

 under hard external conditions, 

 or if any do, they go quickly to 

 ground. 



5. The struggle for existence, 

 and the selection that goes hand 

 in hand with it, constitute a fac- 

 tor which limits new forms and 

 hinders further variation and is, 

 therefore, in no way favourable 

 to the origin of new forms. It 

 is a factor inimical to evolution. 



6. If there were no struggle 

 for existence, there would be no 

 killing out of newly arising or al- 

 ready arisen forms. The world 

 of organisms could then grow to 

 a mighty tree, whose branches 

 could all persist in blossoming 

 condition, and the most aberrant, 

 now isolated, species would be 

 connected with all others through 



intermediate forms. 



7. The adaptation which 

 comes to exist through the 

 struggle for existence is not at 

 all identical with an advance, for 

 higher, more specialised (voll- 

 kommcncrc} forms are by no 

 means always better adapted to 

 outer conditions than the lower 

 ones. One cannot explain the 

 evolution of organisms in a 

 purely mechanical way. In or- 

 der to explain the origin of 

 higher forms out of lower it is 

 necessary to admit a special 

 tendency, in organisms, for ad- 

 vance, which is nearly related 

 to, or identical with the tend- 

 ency to vary, and which com- 

 pels organisms toward perfect- 

 ness as far as external conditions 

 allow. 



The theory of heterogenesis as formulated by Korschinsky 

 (and also as held by de Vries, as we shall see) is not neces- 



6. If there were no struggle 

 for existence, no selection, no 

 survival of the strongest, there 

 would be no evolution and no 

 specialisation, for adapted spe- 

 cies would have no advantage 

 over unadapted ones, and as a 

 result of crossing with the lat- 

 ter, they would sacrifice their 

 useful characteristics. 



7. The so-called advance in 

 nature or the perfecting of or- 

 ganisms, is nothing else than a 

 more complex, more complete 

 adaptation to outer conditions, 

 and it is reached in a purely 

 mechanical way through selec- 

 tion and the accumulation of 

 characteristics useful under the 

 existent outer conditions. 



