254 



THE EVOLUTION THEORY 



been proved in regard to heterostylism by Strasburger ; but in any 

 case it was possible for Nature, by means of slight variations in the 

 characters of the male and female parts of the flower, to diminish the 

 certainty of the meeting of the two germ-cells, even to the total exclu- 

 sion of the possibility of any union of these. 



If, then, self-fertilization had to be guarded against or at least 

 rendered difficult because its consequences were injurious, all variations 

 pointing in the direction of safeguarding would necessarily be pre- 

 served and increased. In many cases variations in the structure 

 of the flower were sufficient ; but when, as in Corydalis cava, the pollen 

 could not readily be prevented from falling upon the stigma, the 

 pollen might be made sterile as far as its own flower was concerned 

 by a process of selection, in which on an average those plants would 



Fig. 128. Heterostylism {Primula sinensis), after Noll. Two heterostylic 

 flowers from different plants. L, the loiig-styled form. K, the short-styled 

 form. G, style. S, anthers. P, p, pollen-grains. N, n, stigmatie papillse of 

 the long-styled and short-styled forms respectively. P, p, N, n, magnified 

 no times. 



remain successful which produced the largest number of cross-fertilized 

 seeds, and in this case those which did so were those whose pollen 

 reacted most feebly to the stimulus of their own stigma. 



That self-sterility in all these different degrees is not a primary 

 character of the species, but an adaptation to the advantages of cross- 

 fertilization, is apparent — if indeed it seems doubtful to any one — 

 especially from cases of heterostylism. I refer to the dimorphism 

 and trimorphism which Darwin discovered in many flowers, and 

 which shows itself in the fact that flowers otherwise almost exactly 

 alike, as, for instance, primroses, may exhibit a long style in some 

 individuals, and in others a short one (Fig. ia8). At the same 

 time, there is a difference in the position of the stamens, which are 

 placed higher up in flowers with short styles, and much lower down 



Digitized by Microsoft® 



