CAUSE OF HYBRID VIGOR 1G5 



proved beyond question tlie ^a'eat advantage to be gainLxi 

 by crossing even when the individuals themselves were 

 weak. These facts, taken together with the many mar- 

 velous and intricate contrivances of phuits to insure 

 cross-pollination, led him to believe that selt'-fertilizatioii 

 was inherently harmful and something to be avoided it" 

 possible. The benefits accming Trom crossing he ascrilied, 

 as we have seen, to the meeting of sexual elements liaving 

 diverse constitutions. 



After Darwin's contribution to the problem of inbree<l- 

 ing no progress was made until less than two decades ago, 

 when the Mendelian discovery opentsl up s(j many new 

 possibilities. The conception of an inheritance made up 

 of separable units aroused a new interest in the matter 

 and made possible a unilied and satisfactory interpreta- 

 tion of all the facts. 



Mendel had shown that characters from one i»arent 

 might disappear completely in the progeny only to reap- 

 pear in subsequent generations in some of the offspring. 

 Surely here was something of importanee to the inbreed 

 ing problem. Unfavorable charaeters might vanish when 

 different organisms were crossed; but they were merely 

 hidden. Inbreeding revealed thom for what they were. 



Shortly after Menders experiments became known. 

 Bateson crossed two pure white Howered varieties of tlie 

 sweet pea. Instead of having the whiti^ flowers oharac 

 teristic of two parental races, the hybrid llowers wer*^ 

 purple. The wild progenitor of tlie sweet pea has purj*!^' 

 flowers. Here was a case in which crossing brought back 

 previously existing conditions, a return to the wild iy\^o 

 characters. This phenomenon had been obser\'ed long 

 before this time; in fact, it was so well known it had been 



