346 SELF-STERILE PLANTS. CHAP. IX. 



between the sterility of the parent-plants when self- 

 fertilised, and the extent to which their offspring suffer 

 in vigour by this process; and some such correspon- 

 dence might have been expected if self-sterility had 

 been acquired on account of the injury caused by self- 

 fertilisation. The fact of individuals of the same 

 parentage differing greatly in their degree of self- 

 sterility is likewise opposed to such a belief; unless, 

 indeed, we suppose that certain individuals have 

 been rendered self-sterile to favour intercrossing, 

 whilst other individuals have been rendered self- 

 fertile to ensure the propagation of the species. The 

 fact of self-sterile individuals appearing only occa- 

 sionally, as in the case of Lobelia, does not counte- 

 nance this latter view. But the strongest argument 

 against the belief that self-sterility has been . acquired 

 to prevent self-fertilisation, is the immediate and 

 powerful effect of changed conditions in either causing 

 or in removing self-sterility. We are not therefore 

 justified in admitting that this peculiar state of the 

 reproductive system has been gradually acquired 

 through natural selection ; but we must look at it as 

 an incidental result, dependent on the conditions to 

 which the plants have been subjected, like the ordinary 

 sterility caused in the case of animals by confinement, 

 and in the case of plants by too much manure, heat, &c. 

 I do not, however, wish to maintain that self-sterility 

 may not sometimes be of service to a plant in preventing 

 self-fertilisation ; but there are so many other means 

 by which this result might be prevented or rendered 

 difficult, including as we shall see in the next chapter 

 the prepotency of pollen from a distinct individual 

 over a plant's own pollen, that self-sterility seems aD 

 almost superfluous acquirement for this purpose. 

 Finally, the most interesting point in regard to self- 



