118 INBREEDING AND OUTBEEEDING 



tion of the problem of inbreeding was lacking. Mendel's 

 work was yet unrecognized ; the principles of inheritance 

 of separate characters, of segregation, of chance recom- 

 bination, Darwin was not permitted to know. Had he 

 realized the way in which recessive characters can be con- 

 cealed for many generations without making their ap- 

 pearance until homozygosity was brought about by in- 

 breeding, doubtless his views on the subject would have 

 been materially changed. 



As we have just indicated, and as we shall have occa- 

 sion to emphasize again, the greatest advance in our 

 knowledge of the significance of inbreeding has come 

 through linking its effects with Mendelian phenomena. 

 The first experiments on the subject made in the light 

 of this discovery were those of G. H. Shull and of East, 

 undertaken independently in 1905 with maize, an ideal 

 cross-fertilized species, as the subject. 



Shull 's investigations were not begun with the object 

 of studying the effects of self-fertilization, but the studies 

 having involved parallel cultures of cross-pollinated and 

 self -pollinated lines, it was impossible not to have noticed 

 the smaller stalks and ears and the greater susceptibility 

 to attacks of the corn-smut (Ustilago maydis] shown by 

 the latter. Interest thus aroused, data were collected 

 bearing on the subject of inbreeding, and in 1908 his first 

 conclusions on the subject were published. 



His observation that the progeny of every self-fer- 

 tilized maize plant is inferior in size, vigor and productive- 

 ness to the progeny of a normal cross-bred plant derived 

 from the same source, corroborated preceding investi- 

 gations made by Morrow and Gardner 152 ' 153 and 



