322 HOW CBOPS 6K0W. 



ized and cross-fertilized plants in the 1st, 3d, and .9th 

 generations were respectively as 64 to 100 ; 35 to 100, 

 and 26 to 100. 



In other cases, but, so far as observed, much less com- 

 monly, self-fertilization gives the best results both as 

 regards numbers and vigor of ofEspring. In Darwin's ex- 

 periments, a variety o.i Mimulus luteus originated, of 

 which the self-fertilized progeny surpassed the cross-fer- 

 tilized, during several generation s. In the seventh gen- 

 eration the ratio of superiority of the self-fertilized, as 

 regards numbers of fruit, was as 137 to 100, and in respecc 

 to size of plants as 136 to 100. 



Continued self -fertilization, is thus limited by its ten- 

 dencj^, as statistically determined, to reduce both the 

 vegetative and reproductive vigor of the plant. On the 

 other hand, cross- fertilization is possible or practicable 

 only within very narrow bounds, and the -increased pro- 

 ductiveness that follows it soon reaches a limit, as is 

 shown by the history of vegetable hybrids. 



That neither mode of fertilization is exclusively or speci- 

 ally adapted to the highest development of plants in gen- 

 eral, or of particular kinds of plants, is shown by the fact 

 that in the cotirse of Darwin's researches on the Ipomea 

 purpurea, just referred to, in the sixth generation a self- 

 fertilized plant (variety) appeared, which was superior to 

 its crossed, collateral, and was able to transmit its vigor 

 . and fertility to its descendants. ;, 



It is evident, therefore, that the causes which lead to 

 higher development co-operate most fully, sometimes in 

 the one, sometimes in the other, mode of impregnation 

 and do not necessarily belong to either. We must be- 

 lieve that excellence in offspring is the result of excel- 

 lence in. the parents, no matter what lines their heredity 

 may have followed, except as these lines have influenced 

 their individual excellence. That crossing commonly 

 gives better offspring than in-and-in breeding is due to 



