GENE MUTATIONS AND EVOLUTION 59 



they also produce the changes on which evolutionary progress 

 is based. 



But more probative than this are two other lines of evi- 

 dence. One lies in the observed effects of gene mutations on 

 organisms. The other lies in an experimental analysis of the 

 nature and causes of mutational change; in the recent uncov- 

 ering of some of the hidden processes occurring at mutation, 

 showing just what is happening. We shall look into both 

 of these. 



From the first it has been observed that when carefully 

 examined most mutations are discovered to be defects. The 

 organism is the worse for their occurrence. They are not of 

 such a nature as to yield adaptations, but the reverse. The 

 changes that they represent are not progress, but the opposite. 



These relations are shown in two ways, each of great 

 importance. First, the specific effect of the mutation on some 

 particular part or function of the organism is, in most cases, 

 to bring about a distinct loss, or an abnormality. Certain 

 mutations cause the body to be distorted, or imperfect in 

 parts, or to lose certain typical structures. Others make the 

 limbs abnormal or small, or result in their total loss. By 

 other mutations the eyes are made imperfect, or very small, or 

 are totally lost. All these are well known and typical results 

 of gene mutation : great numbers of such cases could be cited. 

 In general it is true that a part that has been altered by a 

 mutation is not so well adjusted to the rest of the organism, or 

 to the surrounding conditions, as it was before the mutation 

 occurred. Whether there are any exceptions to this we con- 

 sider later. But certainly the general impression from a group 

 of mutated organisms is one of defectiveness and abnormality. 



Second, and perhaps still more significant for the nature of 

 mutations, is the fact that, in addition to their specific effects 

 on particular parts of the body, mutations usually have a 



