96 THE PROBLEMS OF EVOLUTION 



suits. When change occurs in the genes in a single 

 generation, since the new genes spring from a 

 common source, it must be due to some modifying 

 condition, perhaps not in the external environ- 

 ment, but certainly in the body. The view that 

 all evolutionary change has proceeded from re- 

 combination and selection of existing characters is 

 neither adequate to explain the origin of species 

 nor compatible with the commonly accepted 

 monophyletic origin of living things. In spite of 

 its obvious value it must be associated with the 

 possibility of actual genetic change. 



Absolute scientilBc evidence of the modification 

 of the genes through normal reaction to environ- 

 mental conditions is not available. The idea that 

 all such changes in organisms have come about in 

 response to cosmic radiations is scarcely acceptable, 

 in view of the fact that they constitute such a mi- 

 nute part of the total environment. Nevertheless 

 the proved modification of genes as a result of such 

 influences lends color to the possibility that they 

 may be amenable to other conditions as well, and 

 the possibility of modification of genes through 

 changes in the environment generally must be 

 accepted as a working hypothesis until it is either 

 proved or disproved. Our failure to produce them 

 more freely is not surprising, for changes in the 

 environment are compensated by the body, changes 

 in the body are still further compensated by the 



