224 Experimental Zoology 



Evolution by Means of Definite Variation 



The evidence which we now possess indicates, with some 

 probability, I think, that in some cases, at least, the process of 

 evolution may have been by definite or discontinuous changes 

 from one fixed form to another fixed form. The essence of the 

 process is not that the change has been marked or great, but 

 rather that the new type is, from its first appearance, a definite 

 step in a new direction. What has been gained does not need 

 to be maintained by any process of selection, but becomes a 

 part of the permanent inheritance. It is probable that it may 

 be found convenient to distinguish between different kinds of 

 mutations. In fact, de Vries has already distinguished between 

 mutations that involve only a loss of a character (leading to the 

 formation of a "variety" a varietal or retrogressive mutation), 

 and mutations that are new and form new elementary species 

 progressive mutations; and mutations that are due to a latent 

 character reappearing degressive mutations. Of greater im- 

 portance for the theory of evolution would be the distinction 

 between those progressive mutations that become recessive in the 

 first generation, when crossed with the parent forms, and subse- 

 quently split according to the Mendelian proportion, recessive 

 mutations; those mutations that dominate in the first generation 

 after a back-cross, and may or may not split subsequently, 

 dominant mutations; and those that produce a new permanent 

 form or fixed hybrid, when crossed with the parent hybrid 

 mutations. Any one of these three kinds of mutations may give 

 the starting point for a new group of organisms, a new species, 

 or a new variety, but the chance that they may do so will be dif- 

 ferent in different cases, and this important consideration must 

 next be examined. 



It is recognized that one of the greatest difficulties that Dar- 

 win met with in his theory of natural selection was the swamp- 

 ing effects of crossing. If a new and better fluctuation were 

 to appear, it would, in most cases, in order to perpetuate itself, 

 have to unite with the parent form. It will be recalled that 



