84 GENETIC VARIATIONS 



ture of the genetic system that were described in our second 

 chapter. 



Indeed, if this be the state of the case the history of these 

 supposed gene mutations is but a continuation of the earlier 

 history of supposed mutations, including most of the "muta- 

 tions" of De Vries. These had a period of flourishing as the 

 fundamental steps in evolution, but were later found to be 

 only the result of regrouping and rearrangements among the 

 chromosomes or parts of the chromosomes/^ And now, if 

 they turn out to be position effects the "gene mutations" of 

 Morgan are found to be of the same type ; they are rearrange- 

 ments of small extent and often of complex character. They 

 do not supply the material for constructive or progressive 

 evolution.^" 



If on the other hand the mutations are rather injuries than 

 position effects, the case for their role in progressive evolution 

 is not better. In this case they are essentially disintegrative in 

 character, and cannot be expected to give constructive or 

 progressive results. 



Taking the evidence all together, the case is strong for the 

 essentially disintegrative nature of the observed gene muta- 

 tions. It weighs heavily against their constructive or progres- 

 sive character. It argues strongly against the possibility of 

 explaining by them the adaptive and progressive features of 

 the evolutionary process. The overwhelming majority of 

 mutations, it is clear, are certainly not advantageous. Many 

 of them cause death. Many cause a weakening of the consti- 

 tution. Many produce obvious defects and abnormalities. 

 Experimentation shows that most of them are reductional in 

 their action, while those few that have positive action diverse 

 from the normal are among the most distinctly injurious of 

 mutations. And now, to complete the case, comes the demon- 

 stration that typical "gene mutations" are the result of breaks 



