314 PROBI<EM OF THE ORIGIN OF SPECIES. 



modal individuals survive and reproduce the species. / have failed utterly 

 to discover in these beetles evidence that mutants have taken any great part 

 in evolution, all evidence shozving them to he most rigorously exterminated 

 hy natural selection {Chapter IV). On the other hand, the study of geo- 

 graphical distribution and variation gives the strongest of circumstantial 

 evidences for direct rapid transformation in response to environmental stimuli 

 as the result of dispersion. I am therefore of the opinion that the evolution 

 of the genus Leptinotarsa, and of animals in general, has been continuous 

 and direct, developing new species in migrating races by direct response 

 to the conditions of existence. In this evolution natural selection has acted 

 to determine antecedent states and the persistence of new variations, but 

 in each race or species it acts as the conservator of the race, keeping down 

 extreme variations through their elimination in hibernation, larval life, and 

 selective mating. It is in Leptinotarsa the conservator of the racial mean 

 and mode, the destroyer of all variation diverging much from the ortho- 

 genetic trend of evolution, which is itself a product of natural selection. 



The evolution of the genus Leptinotarsa, according to data gathered, has 

 been through response to the stimuli of the conditions of existence in changed 

 germ-plasm constitution, according to the method of trial and error, with 

 natural selection acting as the conservator of the race by limiting the varia- 

 tions to a narrow range of possibilities. The breeding "mutants" in our gar- 

 dens and laboratories can not tell us how they would succeed in nature ; my 

 experience with these beetles is that they fare badly, and, as far as I can dis- 

 cover, that they play a minor role in the evolution of species. This view 

 differs, therefore, from that of De Vries, who sees in "mutants" the origin 

 of species; the real test is, as De Vries clearly sees, the fate of these 

 "mutants" in nature. This I have been able in some measure to test, and at 

 present there seem to be insuperable difficulties in the path of all observed 

 "mutants" in Leptinotarsa. I therefore regard mutations as prophetic varia- 

 tions indicating what may perhaps be the next species in the evolution of the 

 race. How this evolution is brought about in nature is a subject for observa- 

 tion and experiment, and not for anticipation. The micthod of the origin of 

 variation herein developed does account for the origin of variation upon a 

 natural basis, and "mutation" is shown to be but part of the general phenom- 

 ena of variability. That is, in variability there is unity and not discontinuity, 

 and inheritable variations differ not in kind, but only in degree. 



