PLANT HYBRIDIZATION BEFORE MENDEL 227 



or variety increases the vigour and fertility of the offspring, and on 

 the other hand that very close interbreeding lessens their vigour and 

 fertility, that I cannot doubt the correctness of this conclusion." (2a, 



"Again, both with plants and animals, there is the clearest evidence 

 that a cross between individuals of the same species, which differ to a 

 certain extent, gives vigour and fertility to the offspring; and that close 

 interbreeding continued during several generations between the nearest 

 relations, if these be kept under the same conditions of life, almost al- 

 ways leads to decreased size, weakness, or sterility." (la, 2:27-8.) 



In "Cross and Self-Fertilization," Darwin again discusses the 

 effects of crossing as follows, expressing the view : 



"Firstly, that the advantages of cross-fertilization do not follow from 

 some mysterious virtue in the mere union of two distinct individuals, 

 but from such individuals having been subjected during previous genera- 

 tions to different conditions, or to their having varied in a manner com- 

 monly called spontaneous, so that in either case their sexual elements 

 have been in some degree differentiated; and secondly, that the injury 

 from self-fertilization follows from the want of such differentiation in 

 the sexual elements." (lb, p. 448.) 



"After plants have been propagated by self-fertilization for several 

 generations, a single cross with a fresh stock restores their pristine 

 vigour and we have a strictly analogous result with our domestic ani- 

 mals." (lb, p. 444.) 



"A cross with a fresh stock, or with another variety, seems to be always 

 beneficial whether or not the mother plants have been intercrossed or 

 self-fertilized for several previous generations." (lb, p. 449.) 



Darwin also remarks upon the greater power of the cross- 

 fertilized plants in his experiments to stand exposure, the crossed 

 plants enduring sudden removal from greenhouse to out-of-doors 

 conditions better than did the self-fertilized, and also resisting 

 cold, and intemperate weather conditions more successfully. This 

 was the case with morning-glory and. with Mimulus. 



"The offspring of plants of the eighth self-fertilized generation of 

 Mimulus, crossed by a fresh stock, survived a frost which killed every 

 single self-fertilized and intercrossed plant of the old stock." (lb, 

 p. 289.) 



"Independently of any external cause which could be detected, the 

 self-fertilized plants were more liabre to premature death than the 

 crossed." {ib., p. 290.) 



Out of several hundred plants in all, involved in the experi- 

 ment, only seven of the crossed plants died, while at least twenty- 

 nine of the self-fertilized were thus lost. 



With regard to time of flowering, in four out of fifty-eight 

 cases a crossed, in nine cases a selfed, plant flowered first. 



