394 Evolution by Jumps 



functions, thousands of generations of the kind of eUmina- 

 tion which Wallace and Darwin assumed, but only with 

 respect to significant features, not to those that distinguish 

 the various species from one another for the casual observer. 



How Mutations Are Brought About 



Evolution by mutation would mean the occasional 

 introduction, into the germ plasm of a line of plants or 

 animals, of new factors capable of influencing the develop- 

 ment of characters already present in the species. 



Advantageous characters would presumably survive 

 and become assimilated in the constitution of the strain or 

 race. The new type would become thereby established, and 

 would serve as the basis for still further mutations. Each 

 individual is thus the potential jumping-off point for new 

 possibilities in evolution. This development of de Vrie<:' 

 theory and of the experimental studies of the last thirty 

 years is in accord with Weismann's speculative ideas about 

 the germinal sources of hereditary changes, although Weis- 

 mann himself remained a staunch advocate of natural 

 selection to the end. 



Germinal changes, that is, changes in the chromosome 

 and in the genes, may bring about a new trait not only in 

 every part of the body, but at any stage in development. 

 It might result in increased growth or in smaller stature; 

 in prolonging the duration of life or in shortening it; in 

 advancing the age of maturity or in accelerating the de- 

 velopment. A germinal change might affect a character 

 that appears early in infancy, or one that is manifest only 

 after maturity. Recent studies in Europe have shown that 

 certain qualities of the human voice are inherited in Men- 

 delian fashion; but these characters appear only after 

 puberty. 



Whatever the cause of such germinal changes may turn 

 out to be, it is not necessary to think of mutations as add- 

 ing or subtracting such a very small detail as to leave the 



