82 ESSAYS. 
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. ach 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 ease, 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 physiologist 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. 
