CURRENT LITERATURE 
NOTES FOR STUDENTS 
Hybrid vigor.—The phenomenon of hybrid vigor has come to hold a very 
important place in practical plant breeding, and is of considerable theoretical 
interest to geneticists. The most generally accepted interpretation has been 
East’s' “‘heterozygosis,” according to which hybrids are vigorous because of 
their heterozygous sets. Heterozygosis has been very valuable in helping to 
organize our ideas on the general subject of hybrid vigor, but as a theoretical 
explanation of the phenomenon involved it has been unsatisfactory. When 
one says that hybrids are vigorous because of their heterozygous sets, he is 
making an accurate restatement of the fact of hybrid vigor in the language of 
genetics, but he is not providing any real explanation of the phenomenon. 
The only acceptable “real” explanation that has yet been presented is as 
ollows. In nature a “struggle for existence” occurs among species and indi- 
viduals. There occurs also a struggle for existence among unit characters. If 
a unit character is undesirable it is eliminated, since the species possessing it is 
eliminated. The unit characters, 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 asso- 
ciation with the ‘desirable’ characters. The majority of unit characters 
today, however, may certainly be regarded as “desirable” ones, and a majority 
is sufficient for the present argument. 
The question then is raised as to what constitutes a so-called “desirable” 
character. It may, of course, be any one of a number of things, but is there not 
some feature common to all such ‘‘desirable’”’ characters? The character 
would seem to be vigor. Each ‘‘desirable” character must add somewhat to 
the vigor of the plant that contains it, and if vigor is increased, such things as 
size and productiveness be increased. Those plants, therefore, = 
be most vigorous which have in combination the greatest number of ‘desirable’ 
characters. 
The next question is, what plants, in general, have in combination the 
greatest number of desirable characters? The answer is hybrids, for they com- 
bine the ‘“‘desirable” characters of both parents. Thus, in general, hybrids 
have twice as many “desirable” characters as do pure races. At this point the 
objection is raised that though hybrids do actually contain this double quota, 
* East, E. M., and Haves, H. K., Heterozygosis in Ae and in plant breed- 
ing. U.S. Dept. Keke Bur. Pl. Ind. Bull. no. 243. pp. 58. 
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