22 Mutations and Evolution. 



method of explanation by splitting up the factor could be applied 

 to reversions in other factors which show no such variability as 

 bar-eye. 



A chemical reversal seems more likely to supply a general 

 explanation of reverse mutations, and this is important in its 

 bearing on the nature of a mutational change. For if a recessive 

 mutant factor can revert to the wild condition, then the mutation 

 was not due to an irrevocable loss of a particle, but rather to the 

 transformation of a particle or locus of a chromosome, first in one 

 direction and afterwards back to the original condition. 



As regards the infrequency of simple Mendelian factor 

 mutations in (Enothera, it would appear that this is partly due to 

 their presence being masked by the greater number of lethal 

 factors. And since lethal factors must produce non-viable gametes 

 or zygotes, i.e., sterility of pollen, ovules or seeds, this is in accord 

 with the large amount of sterility in (Enothera. For this reason, 

 except in the case of mutations involving visible changes in the 

 chromosomes, we can seldom be sure in (Enothera that the 

 germinal change in a locus of a chromosome which marks the 

 origin of a mutation, did not happen several or many generations 

 previously. It is this contingency with regard to various mutations 

 that has made it possible to suggest that the phenomena are merely 

 the result of the splitting out of factors acquired in some previous 

 hypothetical cross. This argument has been used, notwithstanding 

 the fact that mutations occur in such well-authenticated and self- 

 pollinated species as (E. bienms Linn. 



It is an interesting ^and significant fact that although 

 Drosophila, like most animals, can only be perpetuated by a 

 crossing of two individuals in every generation, while in such 

 species as CE. biennis natural crossing is a very rare event at best, 

 yet the bogey of hybridization which is so often raised as a 

 complete explanation of the mutations in (Enothera, has never, so far 

 as I am aware, been suggested for Drosophila. Probably the 

 reason is that in Drosophila the evidence is clear that most of the 

 mutations at any rate have had their beginning in the germ plasm 

 at the time they make their external appearance. A great deal of 

 ink would have been spared if it had been recognised that for 

 plants as for animals, for (Enothera as for Drosophila, mutation is 

 a process sui generis, a " spontaneous " disintegration or alteration of 

 elements in the germ plasm which finds certain physical parallels 

 or analogies in the behaviour of the atom of radium and other 



