MEANS AND METHODS OF EVOLUTIONARY CHANGE 337 



different from that of the unchanged gene. In the fruit fly, Drosophila, for 

 example, a dominant gene producing red eye color undergoes mutation, 

 changing to a recessive gene which fails to produce the necessary pigment. 

 Eyes of flies homozygous for this new gene remain white. We should note 

 carefully that the primary change occurs in the genes. As a result of a 

 change in a gene a change is later found in the bodies of all offspring con- 

 taining the altered gene if the latter is dominant, or of all offspring homo- 

 zygous for the altered gene if the latter is recessive. Since they arise as 

 changes in genes, all mutations are, by definition, inheritable. 



Spontaneously occurring, inheritable changes were known long before 

 we knew anything of genes. Darwin mentioned "sports" of this kind in his 

 Origin of Species, including among them the Ancon ram, a male sheep 

 which appeared in the flock of Seth Wright, a Massachusetts farmer, in 

 1 79 1 . Its legs were much shorter than those of other sheep. This characteris- 

 tic proved an advantage to the farmer, since short-legged sheep were less 

 able to scale the stone-wall fences of the day than were other sheep. The 

 ram passed on the new characteristic to his offspring and thereby became 

 the sire of a short-legged race of sheep once common in New England. Here, 

 then, was the appearance of a new, inheritable characteristic. While Darwin 

 recognized that such "sports" occurred he placed little importance upon 

 them, believing them to be too rare to be of much sionificance. 



It remained for the Dutch botanist De Vries to focus attention upon the 

 importance of mutations. Studying the evening primrose, Oenothera la- 

 marckiana, De Vries observed that in a population inhabiting a field near 

 Amsterdam individuals occasionally appeared which differed markedly from 

 their fellows. Some were larger, some smaller; there were differences in 

 flowers, in leaves, and in many other characteristics. When De Vries grew 

 offspring from seeds produced by these unusual plants he found that the 

 characteristics were inherited and that consequently new varieties of several 

 kinds were produced. Here, then, was a species of plant which seemed to 

 be caught in the act of producing new varieties or, as he called some of 

 them, "elementary species." On the basis of these studies De Vries advanced 

 his "mutation theory" of evolution. While later investigation has revealed 

 that the phenomenon of occurrence of mutations does not in itself constitute 

 a complete theory of evolutionary change, De Vries deserves much credit 

 for focusing attention upon such inheritable changes in structure. 



Subsequent to the investigations of De Vries many mutations have been 

 discovered in other plants and in animals. Most of our modern science of 

 genetics grew out of analysis of the mechanisms involved in inheritance 

 of mutations. Details may be sought in books dealing with genetics; it 

 must suffice us to note some of the attributes of mutations which render 



