CHAP. IX. SELF-STERILE PLANTS. 345 



the full fertility of the parent-plants and for the full 

 vigour of their offspring. It seems also probable that 

 with those plants which are capable of complete self- 

 fertilisation, the male and female elements and organs 

 already differ to an extent sufficient to excite their 

 mutual interaction ; but that when such plants are 

 taken to another country, and become in consequence 

 self-sterile, their sexual elements and organs are so 

 acted on as to be rendered too uniform for such inter- 

 action, like those of a self-fertilised plant long 

 cultivated under the same conditions. Conversely, we 

 may further infer that plants which are self-sterile in 

 their native country, but become self-fertile under 

 changed conditions, have their sexual elements so acted 

 on, that they become sufficiently differentiated for 

 mutual interaction. 



We know that self-fertilised seedlings are inferior in 

 many respects to those from a cross ; and as with 

 plants in a state of nature pollen from the same 

 flower can hardly fail to be often left by insects or by 

 the wind on the stigma, it seems at first sight highly 

 probable that self-sterility has been gradually acquired 

 through natural selection in order to prevent self- 

 fertilisation. It is no valid objection to this belief 

 that the structure of some flowers, and the dichogamoua 

 condition of many others, suffice to prevent the pollen 

 reaching the stigma of the same flower ; for we should 

 remember that with most species many flowers 

 expand at the same time, and that pollen from the 

 same plant is equally injurious or nearly so as 

 that from the same flower. Nevertheless, the belief 

 that self-sterility is a quality which has been gradually 

 acquired for the special purpose of preventing self- 

 fertilisation must, I believe, be rejected. In the 

 first place, there is no close correspondence in degree 



