146 GENEEAL RESULTS. CHAP. XII. 



differs in the individuals of the same species. The rela- 

 tive period at which the sexual elements in the same 

 flower are mature, differs in the varieties of Pelar- 

 gonium ; and Carriere gives several cases,* showing 

 that the period varies according to the temperature to 

 which the plants are exposed. 



This extraordinary diversity in the means for 

 favouring or preventing cross- and self-fertilisation in 

 closely allied forms, probably depends on the results 

 of both processes being highly beneficial to the species, 

 but in a directly opposed manner and dependent on 

 variable conditions. Self-fertilisation assures the pro- 

 duction of a large supply of seeds ; and the necessity 

 or advantage of this will be determined by the average 

 length of life of the plant, which largely depends on 

 the amount of destruction suffered by the seeds and 

 seedlings. This destruction follows from the most 

 various and variable causes, such as the presence of 

 animals of several kinds, and the growth of surround- 

 ing plants. The possibility of cross-fertilisation de- 

 pends mainly on the presence and number of certain 

 insects, often of insects belonging to special groups, 

 and on the degree to which they are attracted to the 

 flowers of any particular species in preference to 

 other flowers, all circumstances likely to change. 

 Moreover, the advantages which follow from cross- 

 fertilisation differ much in different plants, so that it 

 is probable that allied plants would often profit in 

 different degrees by cross-fertilisation. Under these 

 extremely complex and fluctuating conditions, with 

 two somewhat opposed ends to be gained, namely, the 

 safe propagation of the species and the production of 

 cross-fertilised, vigorous offspring, it is not surprising 



Des Yarietes,' 1865, p. 30. 



