242 
ORGANIC DEATH. 
Nov., 1890. 
towards death, but are killed by changes of environment. 
There is much to be said in favour of this theory. It 
harmonises with the fundamental law of mechanical force, 
and there is nothing in our personal experience of genera and 
species to disprove it. They appear to be permanent and 
unchangeable until some change in their surroundings alters 
their course or stops them altogether. 
But the contrary theory, that organic force is always 
periodic and must necessarily be dissipated at an epoch which 
is only variable within certain limits, has also some weighty 
evidence in its favour. 
Although our experience of genera and species is con¬ 
sistent with the theory that they might continue for ever by 
perpetual reproduction unless destroyed by external changes, 
our experience of individual organisms is that they always and 
necessarily die after living for a certain period which is prac¬ 
tically definite in each species. It can scarcely be maintained 
that individuals are always crushed out of existence by com¬ 
petitors, or killed by changed surroundings. A man on an 
island without other human inhabitants, though he had 
abundant food, would die of old age within a century ; and if 
the period between human birth and death could be considered 
as long enough for fatal changes of environment, this law 
could not apply to the much shorter lives of the smaller 
mammals, and birds, or to those of such insects as live but a 
few days. 
It seems impossible to account for the constancy and the 
regular periodicity of death in all individual organisms, except 
as the result of their intrinsic constitution. The immortality 
attributed by Weismann and others to unicellular organisms 
can be nothing more than specific immortality. The indi¬ 
viduals perish though the species is continued; for, when a 
cell divides into two independent individuals, the original 
individual has ceased to exist. The moment of fission is the 
moment of death. 
. I think it is indisputable that individual life is a periodic 
phenomenon which must terminate regardless of external 
influences. But if this be so, is it probable that species, 
genera, orders and classes are subject to the same law ? It 
may be, of course, that in these cases the periods are so long 
that we cannot actually observe the beginning and ending of 
any one of them, and that even the 4,000 or 5,000 years of 
written history are quite insufficient to include any such 
record, while yet their periodicity might be as distinct as that 
of individual lives to the eye of a less limited observer. Yet 
it seems also possible that the law of periodicity may not 
apply to them, for there is assuredly a wide difference between 
the constitution of an individual and that of a species. 
