82 ESS A YS. 



No one denies, however, that different species may have an 

 habitual period of death ; we only insist that this is not a 

 necessary period. In the course of things, a multitude of dif- 

 ferent accidents conspire to fix a mean limit to the life of man, 

 which, though far below the natural period of death by old age, 

 yet occurs with such regularity, under given conditions, that 

 it is made a matter of calculation. So a particular kind of 

 tree may be liable to certain accidents, which habitually in- 

 sure its destruction within a definite period. A tree of rapid 

 growth generally has a soft and fragile wood, and is therefore 

 especially subject to decay, or to be broken or overthrown by 

 tempests : and the chances of its destruction are fearfully 

 multiplied with the increasing spread and weight of the 

 branches. Each species, too, being somewhat uniformly ex- 

 posed to a particular class of accidents, according to its con- 

 stitution and mode of growth, may consequently exhibit some- 

 thing like an average duration. But death can no more be 

 said to ensue from old age, in such a case, than in that of the 

 ordinary mortality of mankind. The whole tree does not 

 necessarily suffer, like the animal, from the death or amputa- 

 tion of its limbs ; those that remain may be thereby placed, 

 perhaps, in a more favorable condition than before. A tree 

 may certainly be conceived to survive all ordinary accidents, 

 or to be protected against them, and thus to live indefinitely ; 

 while animals, even if shielded from all external injury, must 

 at last succumb to internal causes of destruction, — unavoid- 

 able, because inseparable from their organization. 



by layering ; whereas the roots themselves descend from a great height. 

 When a sufficient number of these collateral trunks are formed to sup- 

 port the whole weight, the central, original stem may decay and dis- 

 appear, as it often does, without affecting the existence of the tree ; 

 which thus increases into a grove, " high over-arched, with echoing walks 

 between," that obviously may endure for an indefinite period. Many 

 such trees are known, of immense magnitude, and doubtless of most ex- 

 traordinary age. But the vegetable ph3 T siologist well knows that these 

 essentially differ from ordinary trees, only in that a portion of the new 

 wood, detached as it were from the branches, forms separate trunks in- 

 stead of adhering throughout to the main trunk and contributing to its 

 increase in circumference. These collateral trunks merely represent the 

 outer and newer layers of ordinary trees, while the main stem represents 

 the old and often decaying centre. 



