Chap. VI. ON HETEROSTTLED PLANTS. 265 



is invariably applied to the stigma. It is obvious that 

 with heterostyled dimorphic plants the two female 

 and the two male organs differ in power; for if the 

 same kind of pollen be placed on the stigmas of the 

 two forms, and again if the two kinds of pollen be placed 

 on the stigmas of the same form, the results are in 

 each case widely different. Nor can we see how this 

 differentiation of the two female and two male organs 

 could have been effected merely through each kind 

 of pollen being habitually placed on one of the two 

 stigmas. 



Another view seems at first sight probable, namely, 

 that an incapacity to be fertilised in certain ways has 

 been specially acquired by heterostyled plants. We 

 may suppose that our varying species was somewhat 

 sterile (as is often the case) with pollen from its own 

 stamens, whether these were long or short: and that 

 such sterility was transferred to all the individuals 

 with pistils and stamens of the same length, so that 

 these became incapable of intercrossing freely; but 

 that such sterility was eliminated in the ease of the 

 individuals which differed in the length of their pistils 

 and stamens. It is, however, incredible that so peculiar 

 a form of mutual infertility should have been specially 

 acquired unless it were highly beneficial to the species; 

 and although it may be beneficial to an individual 

 plant to be sterile with its own pollen, cross-fertilisa- 

 tion being thus ensured, how can it be any advan- 

 tage to a plant to be sterile with half its brethren, 

 that is, with all the individuals belonging to the same 

 form? Moreover, if the sterility of the unions between 

 plants of the same form had been a special acquire- 

 ment, we might have expected that the long-styled form 

 fertilised by the long-styled would have been sterile 

 in the same degree as the short-styled fertilised by 



