392 Evolution by Jumps 



when a germ cell is being formed. There would then result 

 a single egg with the altered gene, or a couple of sperm cells. 

 If the change took place earlier, it might result in the forma- 

 tion of a considerable number of either eggs or sperm cells 

 containing the altered gene. In any case, the odds are 

 against the change ever manifesting a new condition in 

 developed individuals resulting from them unless the gene 

 change ( i ) came very early and produced very many eggs 

 or sperms of the new kind, and (2) was related to a dom- 

 inant trait. 



If the change in the gene makes for a recessive, the 

 new gene would be reproduced in the body cells as well as in 

 the germ cells, but would show nothing in the succeeding 

 generation of developed individuals. It could be multiplied 

 for several generations and not show anything at all in the 

 offspring. When, however, there were so many individuals 

 bearing this recessive gene that males and females carrying it 

 finally mated, the corresponding character would appear as a 

 freak or a mutant. This corresponds to de Vries' statement 

 that a mutational change would be latent for several genera- 

 tions and then appear suddenly in one or several individuals 

 manifesting the new character. What has been latent is a 

 modified gene. This, being recessive, could not bring about 

 any change in the appearance of individuals, although pres- 

 ent as part of the heritage, and carried on from generation 

 to generation in a certain proportion of the population, in 

 accord with Mendel's principles of the purity of the germ 

 and segregation. 



If such a gene mutation is dominant, however, it may 

 manifest itself in the first generation. Whether or not it 

 continues to multiply would then depend upon the num- 

 bers in which it first appeared and upon the relation of the 

 character to living conditions. If it were useful, or even 

 indifferent, it would continue. Otherwise the disadvantage 

 would lead to its extermination. There is no danger of the 

 new gene being swamped in the old sense, since it retains its 

 qualities or capacities in a " pure " state. 



