20 Descent 



local adaptations with slight changes of the 

 average character in separate localities, seem 

 to be of quite normal occurrence. 



A new method of individual selection has 

 been used in recent years, in America, especially 

 by W. M. Hays. It consists in judging the 

 hereditary worth of a plant by the average con- 

 dition of its offspring, instead of by its own 

 visible characters. If this determination of the 

 " centgener power," as Hays calls it, should 

 prove to be the true principle of selection, then 

 indeed the analog}^ between natural and artifi- 

 cial selection would lose a large part of its im- 

 portance. We will reserve this question for the 

 last lecture, as it pertains more to the future, 

 than to our present stock of knowledge. 



Something should be said here concerning 

 hybrids and hybridism. This problem has of 

 late reached such large proportions that it can- 

 not be dealt with adequately in a short survey 

 of the phenomena of heredity in general. It 

 requires a separate treatment. For this reason 

 I shall limit myself to a single phase of the prob- 

 lem, which seems to be indispensable for a true 

 and at the same time easy distinction between 

 elementary species and retrograde varieties. 

 According to Macfarlane's terminology, some 

 crosses are to be considered as unisexual while 

 others are bisexual. The first are one-sided, 



