204 CONCLUDING REMAKES CHAP. VI. 



manner will be nearly or completely lost. He was 

 led to this view by observing that Diptera frequently 

 carried poileu from the long-styled flowers of Hottonia 

 to the stigma of the same form, and that this ille- 

 gitimate union was not nearly so sterile as the corre- 

 sponding union in other heterostyled species. But 

 this conclusion is directly opposed by some other 

 cases, for instance by that of Linum grandiflorum ; 

 for here the long-styled form is utterly barren with 

 its own-form pollen, although from the position 

 of the anthers this pollen 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 dif- 

 ferent. 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 case 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 



