EVOLUTION AND MUTATION 3 



insure almost universal acceptance. The theory of natural 

 selection is one of the means by wliich tliis position has been 

 reached. It is the apphcation of the breeding practice to 

 the phenomena of nature at large. Darwin's theory is 

 often designated as the theory of natural selection. This is 

 however, not the same as the theory of descent. The idea 

 of descent with modification, wliich now is the basis of all 

 evolutionary science, is quite independent of the question 

 as to how, in the single instances, the change of one species 

 into another has actually taken place. The theory of de- 

 scent remains unshaken even if our conception concerning 

 the mode of descent should prove to be in need of revision. 

 Such a revision has become necessary by the gradual 

 development of the study of variability. Darwin has demon- 

 strated that all the individuals of a given species differ from 

 one another to some extent, and that many of these differ- 

 ences increase or lessen their chances of survival. A struggle 

 for life ensues, and, sooner or later, the unfit individuals 

 succumb, thereby leaving the average of the species changed 

 to some sHght degree. Differences between isolated local 

 races afford the means of studying the efficiency of this process 

 of variability and selection. The question arises, however, as 

 to how far this variability may go under the influence of this 

 guidance. Is it Hmited or unhmited ? Can it proceed during 

 centuries and in the same direction, augmenting the differ- 

 ences to any extent, or is it bound by its original average 

 condition, without being able to diverge far from it? Can 

 it produce new characters and new quaUties or is it Hmited 

 to changes of degree in those that already exist? To all 

 these, and many other questions, an answer could not be 

 given at the time of Darwin, the evidence being too incom- 

 plete. It was necessary, however, to make a decision of 

 some. kind and thus it was universally assumed that the 

 changes by which species originate are slow, almost invisible, 



