EVOLUTION IN NATURE AND UNDER DOMESTICATION. 241 



often the exact reverse takes place from what happens in 

 nature. We mean, that not only are these aberrant individuals 

 hindered from being taken up into the multitude of average 

 individuals by repeated crosses, but on the contrary, very 

 many average, typical individuals are taken up into the new 

 speciesi They are mated to the few very good individuals, and 

 their children also and so on. The change of species under do- 

 mestication is not only one by colonization, but actively a 

 change of the whole species under influence of selection. 



It is evident, that we must be very cautious in concluding as 

 to the course of evolution in nature from facts observed in 

 cultivated animals and plants. Ceitainly the two processes are 

 not identical, and from the fact that under cultivation species 

 are changed by selection we may not conclude that they are 

 similarly changed by natural selection in nature. But if we ob- 

 serve the cause of the difference, the existence in nature of an 

 inert, unchangeable body of individuals in every species, and 

 the absence of such a multitude in cultivation, we have further 

 proof for our contention that species are on the whole stable, 

 unchangeable, but that new species differing in small, often 

 adaptative changes from parent-species, are continually being 

 formed in those cases where crossing furnishes the necessary 

 potential variability, and isolation furnishes the chance for the 

 new group of settling down to its own genotype. 



New species can become established only if three conditions 

 are fulfilled. The potential variability of the parent-species 

 must be high enough, the new group must be sufficiently iso- 

 lated from inter-breeding with the parent-species,' or closely re- 

 lated species, and thirdly, the genot5^e for which the new 

 group becomes pure must be such as to insure its members a 

 reasonable chance of surviving and procreating. 



If we consider the possibilities and probabilities of species- 

 formation it becomes apparent, that different groups of organ- 

 isms must have different chances to produce new species, 

 that the course of evolution is not the same in all groups of or- 

 ganisms. Rate of reproduction, rate of dispersal, colonizing hab- 



16 



