GENE MUTATIONS AND EVOLUTION 65 



resulting in weakening of the constitution, so that under 

 ordinary conditions the white-eyed flies show higher mortal- 

 ity than the normal red-eyed. But JoUos^ finds that the white- 

 eyed flies have a greater resistance to high temperatures than 

 the normal individuals with unmutated red eyes. This is, so 

 far as I know, the only case of a mutation held to increase the 

 resistance of the organism to bad conditions. 



Other cases of mutations held to be advantageous could 

 doubtless be culled from the literature. But they are rare, and 

 the evidence they present is not impressive. From them a 

 convincing case cannot be made out for beneficial and adap- 

 tive mutations. 



But with relation to progressiveness, we find it commonly 

 stated as an established fact that not all mutations are reduc- 

 tional, although most of them are. Some of them, it is 

 asserted, are progressive.* But when we look at the examples 

 of such progressive mutations, what do we find ? The exam- 

 ples given are what are known as reverse or return mutations. 

 A normal gene has mutated in the usual reductional manner. 

 In some cases it later returns to the normal condition, and this 

 is called a reverse mutation. As before described, a consider- 

 able number of such reverse mutations have been observed, 

 some occurring spontaneously, others under radiation. Now, 

 if the original mutation was reductional, then, it is urged, the 

 reverse mutation — the return to normal — must be progressive. 

 This is logical, but what is its relation to the question in 

 which we are here interested ? A reverse mutation, it is clear, 

 gives no example of a change producing progressive evolu- 

 tion. It restores the original condition, but does not carry 

 things farther. If mutations are essentially reductions, injur- 

 ies or abnormalities, the return mutation is merely a cure of 

 the original injury. If there are mutations that yield progres- 

 sive evolution, they must be of a different character from this. 



