INBREEDING EXPERIMENTS 117 



uniform conditions, does not benefit the offspring in tlie 

 least or only in a very slight degree. Mimulus and the 

 descendants of Ipomea, named Hero, offer instances of 

 this rule. Again, plants self -fertilized during several 

 generations profit only to a small extent by a cross with 

 intercrossed plants of the same stock (as in the case of 

 Dianthus), in, comparison with the effects of a cross by 

 a fresh stock. Plants of the same stock intercrossed dur- 

 ing several generations (as with Petunia) were inferior 

 in a marked manner in fertility to those derived from 

 the corresponding self -fertilized plants crossed by a fresh 

 stock. Lastly, certain plants which are regularly inter- 

 crossed by insects in a state of nature, and which were 

 artificially crossed in each succeeding generation in the 

 course of my experiments, so that they can never or most 

 rarely have suffered any evil from self-fertilization (as 

 with Eschscholtzia and Ipomea), nevertheless profited 

 greatly by a cross with a fresh stock. These several cases 

 taken together show us in the clearest manner that it is 

 not the mere crossing of any two individuals which is 

 beneficial to the offspring. The benefit thus derived de- 

 pends on the plants which are united differing in some 

 manner, and there can hardly be a doubt that it is in the 

 constitution or nature of the sexual elements. Anyhow, 

 it is certain that the differences are not of an external 

 nature, for two plants which resemble each other as 

 closely as the individuals of the same species ever do, 

 profit in the plainest manner when intercrossed, if their 

 progenitors have been exposed during several genera- 

 tions to different conditions. ' ' 



Unfortunately, in Darwin's time the key to the solu- 



