472 GENERAL EESULTS. CHAP. XII. 



is determined by some cause fundamentally different 

 from that which determines the sterility of the indi- 

 viduals both of ordinary and of heterostyled plants 

 when united in various ways. Nevertheless, I am 

 aware that it will take many years to remove this 

 prejudice. 



There is hardly anything more wonderful in nature 

 than the sensitiveness of the sexual elements to external 

 influences, and the delicacy of their affinities. We see 

 this in slight changes in the conditions of life being 

 favourable to the fertility and vigour of the parents, 

 while certain other and not great changes cause them 

 to be quite sterile without any apparent injury to their 

 health. We see how sensitive the sexual elements of 

 those plants must be, which are completely sterile with 

 their own pollen, but are fertile with that of any other 

 individual of the same species. Such plants become 

 either more or less self-sterile if subjected to changed 

 conditions, although the change may be far from great. 

 The ovules of a heterostyled trimorphic plant are 

 affected very differently by pollen from the three sets 

 of stamens belonging to the same species. With ordi- 

 nary plants the pollen of another variety or merely of 

 another individual of the same variety is often strongly 

 prepotent over its own pollen, when both are placed 

 at the same time on the same stigma. In those great 

 families of plants containing many thousand allied 

 species, the stigma of each distinguishes with unerr 

 ing certainty its own pollen from that of every 

 other species. 



There can bo no doubt that the sterility of dis- 

 tinct species when first crossed, and of their hybrid 

 offspring, depends exclusively on the nature or affi- 

 nities of their sexual elements. We see this in the 

 want of any close correspondence between the degree 



