CAUSE OF HYBRID VIGOR 165 



proved beyond question the great advantage to be gained 

 by crossing even when the individuals themselves were 

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

 velous and intricate contrivances of plants to insure 

 cross-pollination, led him to believe that self-fertilization 

 was inherently harmful and something to be avoided if 

 possible. The benefits accruing from crossing he ascribed, 

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

 diverse constitutions. 



After Darwin 's contribution to the problem of inbreed- 

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

 when the Mendelian discovery opened up so many new 

 possibilities. The conception of an inheritance made up 

 of separable units aroused a new interest in the matter 

 and made possible a unified and satisfactory interpreta- 

 tion of all the facts. 



Mendel had shown that characters from one parent 

 might disappear completely in the progeny only to reap- 

 pear in subsequent generations in some of the offspring. 

 Surely here was something of importance to the inbreed- 

 ing problem. Unfavorable characters might vanish when 

 different organisms were crossed ; but they were merely 

 hidden. Inbreeding revealed them for what they were. 



Shortly after Mendel's experiments became known, 

 Bateson crossed two pure white flowered varieties of the 

 sweet pea. Instead of having the white flowers charac- 

 teristic of two parental races, the hybrid flowers were 

 purple. The wild progenitor of the sweet pea has purple 

 flowers. Here was a case in which crossing brought back 

 previously existing conditions, a return to the wild type 

 characters. This phenomenon had been observed long 

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



