ORIGIN AND SIGNIFICANCE OF SEX. 233 



the fusion of two protoplasmic cells, have in other 

 respects the same surroundings, so that we cannot 

 attribute the lack of development of the former to 

 a lack of nutriment, but must regard it rather as 

 owing to the absence of that stimulus derived from 

 the fusion and mutual influence of two cells of 

 different potentialities, that is, different ancestries. 

 That this latter is important, is shown by the fact 

 that many plants cannot be fertilised by their own 

 pollen, when the same pollen is efficient with other 

 plants. This self-sterility of plants seems most 

 plausibly explained as the result of too close inter- 

 breeding, whereby the sexual elements of the 

 same growth have lost that peculiar differentiation 

 which is necessary to produce the mutual stimulus 

 for development of the germ. Says Darwin, " 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 always 

 leads to decreased size, weakness, or sterility " ; 

 and again, "slight crosses, that is crosses between 

 the males and females of the same species, which 

 have been subjected to slightly different conditions, 



