THE MUTATION THEORY 339 



essary first to consider the proposition — emphasized by East and 

 Jones in their book, "Inbreeding and Cross-breeding" — that the only 

 way for a genetically sound stock to be formed is by its going through 

 a course of inbreeding, with elimination, by natural or artificial selec- 

 tion, of the undesirable individuals that appear in the course of this 

 inbreeding. The truth of this proposition depends upon the fact that 

 many recessive genes of undesirable character are apt to exist in a 

 population. Since the frequency with which these genes are able to 

 produce their characteristic effects, i.e., to "come to light," depends on 

 the closeness of the inbreeding, it is evident that inbreeding will be 

 necessary in order to recognize the genes adequately, and hence to 

 eliminate them. 



Our present theory of mutation, however, carries us further than 

 the proposition just considered. It shows that these undesirable 

 genes have arisen by mutation; in fact, as stated in point 12, the great 

 majority of mutations are deleterious, probably even to the degree of 

 being lethal, and it is also known, as noted in point n, that many — 

 probably the great majority — are recessive. In other words, our 

 mutation theory shows that probably the majority of the mutations 

 that are occurring are giving rise to genes of just the type specified in 

 the above discussion. This immediately shows us that not only are 

 inbreeding and selection desirable for raising the genetic level of a 

 population, but they are absolutely necessary merely in order to main- 

 tain it at its present standard. For the same process of mutation 

 which was responsible for the origination of these undesirable genes in 

 the past must be producing them now, and will continue to produce 

 them in the future. Therefore, without selection, or without the in- 

 breeding that makes effective selection possible, these lethals and other 

 undesirable genes will inevitably accumulate, until the germ plasm 

 becomes so riddled through with defect that pure lines cannot be ob- 

 tained, and progress through selection of desirable recessive traits can 

 never more be effected, since each of them will have become tied up 

 with a lethal. To avoid such a complete and permanent collapse of the 

 evolutionary process, it is accordingly necessary for man or nature to 

 resort to a periodically repeated, although not continuous, series of 

 inbreedings and selections in the case of any biparental organism. 



This conclusion is more than a mere speculation, or even a deduc- 

 tion from our principles. The reality of this process of mutational 

 deterioration has been directly proved, in the case of Drosophila, 

 through experiments that T have conducted on lines in which the 



