No. 507] DEVELOPMENT AND HEREDITY 175 



reconsideration of the work of Darwin and others shows 

 that it accords with this theory. As a single example 

 from Darwin we may consider the experiments with 

 Ipomoea purpurea, which were his longest, being carried 

 on as they were for ten generations. If an actual degen- 

 eration were taking place it might be expected that the 

 difference in height between the crossed and the selfed 

 plants should have gone on increasing in the later genera- 

 tions. Such was not the case, however, and Darwin him- 

 self remarked upon it. Nevertheless, the results ob- 

 tained were what should have been expected by the hy- 

 pothesis just given, for after a few generations even the 

 crossed plants in such a small lot would have become 

 more or less inbred, and would have approached the in- 

 bred stock in size. In further support of this view it is 

 noticeable that the crossed flowers varied in color in the 

 earlier generations, but became more uniform toward the 

 end of the experiment, while the selfed plants were uni- 

 form in color throughout the whole time. This, then, 

 explains why his out-crosses with other stock showed 

 greater vigor than did cross-fertilization within a type, 

 the latter strain having become more nearly identified 

 with the selfed plants through continued close breeding. 



Such results as the above from Darwin, my own ex- 

 periments with maize and tobacco, together with the 

 ready agreement of the facts in other breeding work with 

 which I am familiar, indicate that this problem of de- 

 generation combines two questions ; the one a question of 

 heredity, the other a question of development. Daven- 

 port 4 has recently called attention to what I consider the 



former 



question, suggesting that deterioration through 



inbreeding may be due to the isolation of homozygous 

 recessives, or the combination of recessive di-hybrids. 

 J-his is undoubtedly true when the allelomorphic pair 

 under consideration is the presence and absence of some- 

 thing essential to the normal development of the organ- 



