170 Plant Genetics 



factors; in other words, this is merely a statement that 

 hybrids are vigorous because they are hybrids, with 

 the addition that the more hybrid a hybrid is the more 

 vigorous it is. It follows therefore that heterozygosis is 

 not an explanation of hybrid vigor, but merely a restate- 

 ment of the phenomenon in MendeUan terms, with the 

 additional idea that there may be various degrees of 

 hybrid vigor. It is not the intention to discredit 

 heterozygosis as a valuable conception, but merely to 

 point out that it is not a real explanation, merely a more 

 intelHgent statement of the facts. Restatements should 

 not be confused with explanations. 



A suggested explanation may be considered. In 

 nature a "struggle for existence" occurs among species 

 and individuals. There occurs also a struggle for ex- 

 istence among unit characters. If a unit character 

 is undesirable it is eliminated, for the individual or 

 species that carries it is eliminated. The unit charac- 

 ters, therefore, that have survived and appear in the 

 plants of today are for the most part desirable ones, 

 although some undesirable ones also may have survived, 

 having been carried through in association with the 

 desirable characters. The majority of unit characters 

 today, however, may certainly be regarded as desirable 

 ones, and a majority is sufficient for our present con- 

 sideration. 



The question may be raised as to what constitutes a 

 desirable character. It may be any one of a number of 

 things, but is there not some feature common to all 

 desirable characters? The common feature of all de- 

 sirable characters would seem to be vigor. Each desir- 

 able character must add somewhat to the vigor of 



