66 GENETIC VARIATIONS 



Pause now for a moment to look back at the history of the 

 idea that gene mutations furnish the steps in progressive 

 evolution. You v^^ill recall that for many years, and until 

 comparatively recently, nothing was known as to the agent 

 causing mutations; they appeared to occur "spontaneously," 

 or from inner causes. 



It was observed that most of them — at least most pro- 

 nounced mutations — are injurious in effect. But, it was set 

 forth, this is doubtless to be expected. Assuming that these are 

 undirected changes, it is natural that marked chance altera- 

 tions occurring in a complicated system such as an organism 

 should as a rule be harmful. But among the great number 

 occurring, there will be some few, it was urged, that turn out 

 to be advantageous; it is these that furnish the steps in pro- 

 gressive evolution. Moreover, it could be pointed out that 

 there are great numbers of slight mutations that are not 

 obviously disadvantageous or reductional. This line of 

 argument was plausible, was reasonable; it was generally 

 accepted. 



Something of a blow was given to this conception of the 

 matter when it was found that the agents that produce gene 

 mutations are powerfully injurious; that they kill most of the 

 individuals subjected to them, and that they damage the sur- 

 vivors in all sorts of ways. Particularly remarkable did it 

 appear that they damage the genetic system, breaking up the 

 chromosomes, destroying some of the genes and so producing 

 deficiencies; injuring others so that they will not support 

 development, and others so that they produce inherited losses 

 and abnormalities. It seemed a paradox that among ihese 

 things we should find the progressive changes that give 

 evolutionary advance. But some paradoxical things are true, 

 and the theory stood up under the burden thus laid upon it. 



