iv.] OF THE ANCIENTS. 139 



In animals the frequent union of individuals 

 too nearly identical is checked by the power of 

 locomotion which they possess, and a still further 

 tendency to variation is brought about by the 

 changes in climate, food, &c. which they have to 

 encounter. 



But in plants, special contrivances against self- 

 fertilization appear to have been required for pre- 

 venting the too rapid deterioration of the race, so 

 that, even in cases where the male and female 

 organs grow together, it has been provided, that 

 the pollen of one flower should be conveyed in 

 various ways to the stigmas of another ; and it 

 seems a significant fact, that in so much the greater 

 majority of instances, trees, and other plants of long 

 duration and of vigorous growth, should possess 

 either moncecous or dicecous flowers, as if it were 

 intended by this arrangement to renovate more 

 effectually the vitality of the plant, and thus to 

 secure to the species a longer period of existence. 



Yet with all these provisions for prolonging the 

 life of a species, its days, like those of the indi- 

 viduals composing it, are numbered, and the only 

 question that remains for us to consider is, whether 

 its dying out is to be regarded simply as the result 

 of the altered condition of climate, soil, &c. to 

 which it has been subjected, or occurs from some 

 inherent tendency to decay in its own organization. 



The former explanation is the most obvious one, 

 and may prove satisfactory to certain minds ; for 



