There are three chief kinds of death, (_a_) The great majority of
animals come to a violent end, being devoured by others or killed by
sudden and extreme changes in their surroundings. (_b_) When an animal
enters a new habitat, or comes into new associations with other
organisms, it may be invaded by a microbe or by some larger parasite to
which it is unaccustomed and to which it can offer no resistance. With
many parasites a "live-and-let-live" compromise is arrived at, but new
parasites are apt to be fatal, as man knows to his cost when he is
bitten by a tse-tse fly which infects him with the microscopic animal (a
Trypanosome) that causes Sleeping Sickness. In many animals the
parasites are not troublesome as long as the host is vigorous, but if
the host is out of condition the parasites may get the upper hand, as in
the so-called "grouse disease," and become fatal. (_c_) But besides
violent death and microbic (or parasitic) death, there is natural death.
This is in great part to be regarded as the price paid for a body. A
body worth having implies complexity or division of labour, and this
implies certain internal furnishings of a more or less stable kind in
which the effects of wear and tear are apt to accumulate. It is not the
living matter itself that grows old so much as the framework in which it
works--the furnishings of the vital laboratory. There are various
processes of rejuvenescence, e.g. rest, repair, change, reorganisation,
which work against the inevitable processes of senescence, but sooner or
later the victory is with ageing. Another deep reason for natural death
is to be found in the physiological expensiveness of reproduction, for
many animals, from worms to eels, illustrate natural death as the
nemesis of starting new lives. Now it is a very striking fact that to a
large degree the simplest animals or Protozoa are exempt from natural
death. They are so relatively simple that they can continually
recuperate by rest and repair; they do not accumulate any bad debts.
Moreover, their modes of multiplying, by dividing into two or many
units, are very inexpensive physiologically. It seems that in some
measure this bodily immortality of the Protozoa is shared by some simple
many-celled animals like the freshwater Hydra and Planarian worms. Here
is an interesting chapter in evolution, the evolution of means of
evading or staving off natural death. Thus there is the well-known case
of the Paloloworm of the coral-reefs where the body breaks up in
liberating the germ-cells, but the head-end remains fixed in a crevice
of the coral, and buds out a new body at leisure.
Along with the evolution of the ways of avoiding death should be
considered also the gradual establishment of the length of life best
suited to the welfare of the species, and the punctuation of the
life-history to suit various conditions.
[Illustration: _Photo: J. J. Ward, F.E.S._