344 SELF-STERILE TLANTS. Ciiap. IX. 



in a single generation partially self-fertile, and still 

 more so in the second generation. Conversely, the 

 offspring of English plants, after growing for two 

 seasons in Brazil, became in the first generation quite 

 self-sterile. Again, Ahutilon darwinii, which is self- 

 sterile in its native home of Brazil, became mode- 

 rately self-fertile in a single generation in an English 

 hothouse. Some other plants are self-sterile during the 

 early part of the year, and later in the season become 

 self-fertile. Passijiora alata lost its self-sterility when 

 grafted on another species. With Besecla, however, 

 in which some individuals of the same parentage are 

 self-sterile and others are self-fertile, we are forced in 

 . our ignorance to speak of the cause as due to spon- 

 taneous variability ; but we should remember that the 

 progenitors of these plants, either on the male or 

 female side, may have been exposed to somewhat 

 different conditions. The power of the environment 

 thus to affect so readily and in so peculiar a manner 

 the reproductive organs, is a fact which has many 

 important bearings; and I have therefore thought 

 the foregoing details worth giving. For instance, the 

 sterility of many animals and plants under changed 

 conditions of life, such as confinement, evidently comes 

 within the same general principle of the sexual 

 system being easily affected by the environment. It 

 has already been proved, that a cross between plants 

 which have been self-fertilised or intercrossed during 

 several generations, having been kept all the time 

 under closely similar conditions, does not benefit 

 the offspring ; and on the other hand, that a crosa 

 between plants that have been subjected to different 

 conditions benefits the offspring to an extraordinary 

 degree. We may therefore conclude that some degree 

 of differentiation in the sexual system is necessary foi 



