shortening of life is brought about, our first appeal must be to the
process of natural selection. Duration of life, like every other
characteristic of an organism, is subject to individual fluctuations.
From our experience with the human species we know that long life is
hereditary. As soon as the long-lived individuals in a species obtain
some advantage in the struggle for existence, they will gradually
become dominant, and those with the shortest lives will be exterminated.
So far everything is quite simple; but hitherto we have only considered
the external mechanism, and we must now further inquire as to the
concomitant internal means by which such processes are rendered
possible.
This brings us face to face with one of the most difficult problems in
the whole range of physiology,—the question of the origin of death. As
soon as we thoroughly understand the circumstances upon which normal
death depends in general, we shall be able to make a further inquiry as
to the circumstances which influence its earlier or later appearance,
as well as to any functional changes in the organism which may produce
such a result.
The changes in the organism which result in normal death,—senility
so-called,—have been most accurately studied among men. We know that
with advancing age certain alterations take place in the tissues, by
which their functional activity is diminished; that these changes
gradually increase, and finally either lead to direct or so-called
normal death, or produce indirect death by rendering the organism
incapable of resisting injuries due to external influences. These
senile changes have been so well described from the time of Burdach and
Bichat to that of Kussmaul, and are so well known, that I need not
enter into further details here.
In answer to an inquiry as to the causes which induce these changes in
the tissues, I can only suggest that the cells which form the vital
constituents of tissues are worn out by prolonged use and activity. It
is conceivable that the cells might be thus worn out in two ways;
either the cells of a tissue remain the same throughout life, or else
they are being continually replaced by younger generations of cells,
which are themselves cast off in their turn.
In the present state of our knowledge the former alternative can hardly
be maintained. Millions of blood corpuscles are continually dying and
being replaced by new ones. On both the internal and external surfaces
of the body countless epithelial cells are being incessantly removed,
while new ones arise in their place; the activity of many and probably
of all glands is accompanied by a change in their cells, for their
secretions consist partly of detached and partly of dissolved cells; it
is stated that even the cells of bone, connective tissue, and muscle
undergo the same changes, and nervous tissue alone remains, in which it
is doubtful whether such a renewal of cells takes place. And yet as
regards even this tissue, certain facts are known which indicate a
normal, though probably a slow renewal of the histological elements. I
believe that one might reasonably defend the statement,—in fact, it has
already found advocates,—that the vital processes of the higher (i.e.
multicellular) animals are accompanied by a renewal of the
morphological elements in most tissues.
This statement leads us to seek the origin of death, not in the waste
of single cells, but in the limitation of their powers of reproduction.
Death takes place because a worn-out tissue cannot for ever renew
itself, and because a capacity for increase by means of cell-division
is not everlasting, but finite [See Note 8]. This does not however
imply that the immediate cause of death lies in the imperfect renewal
of cells, for death would in all cases occur long before the
reproductive power of the cells had been completely exhausted.
Functional disturbances will appear as soon as the rate at which the
worn-out cells are renewed becomes slow and insufficient.
But it must not be forgotten that death is not always preceded by
senility, or a period of old age. For instance, in many of the lower
animals death immediately follows the most important deed of the
organism, viz. reproduction. Many Lepidoptera, all may-flies, and many
other insects die of exhaustion immediately after depositing their
eggs. Men have been known to die from the shock of a strong passion.
Sulla is said to have died as the result of rage, whilst Leo X
succumbed to an excess of joy. Here the psychical shock caused too
intense an excitement of the nervous system. In the same manner the
exercise of intense effort may also produce a similarly fatal
excitement in the above-mentioned insects. At any rate it is certain
that when, for some reason, this effort is not made, the insect lives
for a somewhat longer period.
It is clear that in such animals as insects we can only speak
figuratively of normal death, if we mean by this an end which is not
due to accident. In these animals an accidental end is the rule, and is
therefore, strictly speaking, normal [See Note 9].
Assuming the truth of the above-mentioned hypothesis as to the causes
of normal death, it follows that the number of cell-generations which
can proceed from the egg-cell is fixed for every species, at least
within certain limits; and this number of cell-generations, if
attained, corresponds to the maximum duration of life in the
individuals of the species concerned. Shortening of life in any species
must depend upon a decrease in the number of successive
cell-generations, while conversely, the lengthening of life depends
upon an increase in the number of cell-generations over those which
were previously possible.
Such changes actually take place in plants. When an annual plant
becomes perennial, the change—one in every way possible—can only happen
by the production of new shoots, i. e. by an increase in the number of
cell-generations. The process is not so obvious in animals, because in
them the formation of young cells does not lead to the production of
new and visible parts, for the new material is merely deposited in the
place of that which is worn out and disappears. Among plants, on the
other hand, the old material persists, its cells become lignified, and
it is built over by new cells which assume the functions of life.
It is certainly true that the question as to the necessity of death in
general does not seem much clearer from this point of view than from
the purely physiological one. This is because we do not know why a cell
must divide 10,000 or 100,000 times and then suddenly stop. It must be
admitted that we can see no reason why the power of cell-multiplication
should not be unlimited, and why the organism should not therefore be
endowed with everlasting life. In the same manner, from a physiological
point of view, we might admit that we can see no reason why the
functions of the organism should ever cease.
It is only from the point of view of utility that we can understand the
necessity of death. The same arguments which were employed to explain
the necessity for as short a life as possible, will with but slight
modification serve to explain the common necessity of death[4].
Let us imagine that one of the higher animals became immortal; it then
becomes perfectly obvious that it would cease to be of value to the
species to which it belonged. Suppose that such an immortal individual
could escape all fatal accidents, through infinite time,—a supposition
which is of course hardly conceivable. The individual would
nevertheless be unable to avoid, from time to time, slight injuries to
one or another part of its body. The injured parts could not regain
their former integrity, and thus the longer the individual lived, the
more defective and crippled it would become, and the less perfectly
would it fulfil the purpose of its species. Individuals are injured by
the operation of external forces, and for this reason alone it is
necessary that new and perfect individuals should continually arise and
take their place, and this necessity would remain even if the
individuals possessed the power of living eternally.
From this follows, on the one hand, the necessity of reproduction, and,
on the other, the utility of death. Worn-out individuals are not only
valueless to the species, but they are even harmful, for they take the
place of those which are sound. Hence by the operation of natural
selection, the life of our hypothetically immortal individual would be
shortened by the amount which was useless to the species. It would be
reduced to a length which would afford the most favourable conditions
for the existence of as large a number as possible of vigorous
individuals, at the same time.
If by these considerations death is shown to be a beneficial
occurrence, it by no means follows that it is to be solely accounted
for on grounds of utility. Death might also depend upon causes which
lie in the nature of life itself. The floating of ice upon water seems
to us to be a useful arrangement, although the fact that it does float
depends upon its molecular structure and not upon the fact that its
doing so is of any advantage to us. In like manner the necessity of
death has been hitherto explained as due to causes which are inherent
in organic nature, and not to the fact that it may be advantageous.
I do not however believe in the validity of this explanation; I
consider that death is not a primary necessity, but that it has been
secondarily acquired as an adaptation. I believe that life is endowed
with a fixed duration, not because it is contrary to its nature to be
unlimited, but because the unlimited existence of individuals would be
a luxury without any corresponding advantage. The above-mentioned
hypothesis upon the origin and necessity of death leads me to believe
that the organism did not finally cease to renew the worn-out cell
material because the nature of the cells did not permit them to
multiply indefinitely, but because the power of multiplying
indefinitely was lost when it ceased to be of use.
I consider that this view, if not exactly proved, can at any rate be
rendered extremely probable.
It is useless to object that man (or any of the higher animals) dies
from the physical necessity of his nature, just as the specific gravity
of ice results from its physical nature. I am quite ready to admit that
this is the case. John Hunter, supported by his experiments on
_anabiosis_, hoped to prolong the life of man indefinitely by alternate
freezing and thawing; and the Veronese Colonel Aless. Guaguino made his
contemporaries believe that a race of men existed in Russia, of which
the individuals died regularly every year on the 27th of November, and
returned to life on the 24th of the following April. There cannot
however be the least doubt, that the higher organisms, as they are now
constructed, contain within themselves the germs of death. The question
however arises as to how this has come to pass; and I reply that death
is to be looked upon as an occurrence which is advantageous to the
species as a concession to the outer conditions of life, and not as an
absolute necessity, essentially inherent in life itself.
Death, that is the end of life, is by no means, as is usually assumed,
an attribute of all organisms. An immense number of low organisms do
not die, although they are easily destroyed, being killed by heat,
poisons, &c. As long, however, as those conditions which are necessary
for their life are fulfilled, they continue to live, and they thus
carry the potentiality of unending life in themselves. I am speaking
not only of the Amoebae and the low unicellular Algae, but also of far
more highly organized unicellular animals, such as the Infusoria.
The process of fission in the Amoeba has been recently much discussed,
and I am well aware that the life of the individual is generally
believed to come to an end with the division which gives rise to two
new individuals, as if death and reproduction were the same thing. But
this process cannot be truly called death. Where is the dead body? what
is it that dies? Nothing dies; the body of the animal only divides into
two similar parts, possessing the same constitution. Each of these
parts is exactly like its parent, lives in the same manner, and finally
also divides into two halves. As far as these organisms are concerned,
death can only be spoken of in the most figurative sense.
There are no grounds for the assumption that the two halves of an
Amoeba are differently constituted internally, so that after a time one
of them will die while the other continues to live. Such an idea is
disproved by a recently discovered fact. It has been noticed in
_Euglypha_ (one of the Foraminifera) and in other low animals of the
same group, that when division is almost complete, and the two halves
are only connected by a short strand, the protoplasm of both parts
begins to circulate, and for some time passes backwards and forwards
between the two halves. A complete mingling of the whole substance of
the animal and a resulting identity in the constitution of each half is
thus brought about before the final separation [See Note 10].
The objection might perhaps be raised that, if the parent animal does
not exactly die, it nevertheless disappears as an individual. I cannot
however let this pass unless it is also maintained that the man of
to-day is no longer the same individual as the boy of twenty years ago.
In the growth of man, neither structure nor the components of structure
remain precisely the same; the material is continually changing. If we
can imagine an Amoeba endowed with self-consciousness, it might think
before dividing ‘I will give birth to a daughter,’ and I have no doubt
that each half would regard the other as the daughter, and would
consider itself to be the original parent. We cannot however appeal to
this criterion of personality in the Amoeba, but there is nevertheless
a criterion which seems to me to decide the matter: I refer to the
continuity of life in the same form.
Now if numerous organisms, endowed with the potentiality of
never-ending life, have real existence, the question arises as to
whether the fact can be understood from the point of view of utility.
If death has been shown to be a necessary adaptation for the higher
organisms, why should it not be so for the lower also? Are they not
decimated by enemies? are they not often imperfect? are they not worn
out by contact with the external world? Although they are certainly
destroyed by other animals, there is nothing comparable to that
deterioration of the body which takes place in the higher organisms.
Unicellular animals are too simply constructed for this to be possible.
If an infusorian is injured by the loss of some part of its body, it
may often recover its former integrity, but if the injury is too great
it dies. The alternative is always perfect integrity or complete
destruction.
We may now leave this part of the subject, for it is obvious that
normal death, that is to say, death which arises from internal causes,
is an impossibility among these lower organisms. In those species at
any rate in which fission is accompanied by a circulation of the
protoplasm of the parent, the two halves must possess the same
qualities. Since one of them is endowed with a potentiality for
unending life, and must be so endowed if the species is to persist, it
is clear that the other exactly similar half must be endowed with equal
potentiality.
Let us now consider how it happened that the multicellular animals and
plants, which arose from unicellular forms of life, came to lose this
power of living for ever.
The answer to this question is closely bound up with the principle of
division of labour which appeared among multicellular organisms at a
very early stage, and which has gradually led to the production of
greater and greater complexity in their structure.
The first multicellular organism was probably a cluster of similar
cells, but these units soon lost their original homogeneity. As the
result of mere relative position, some of the cells were especially
fitted to provide for the nutrition of the colony, while others
undertook the work of reproduction. Hence the single group would come
to be divided into two groups of cells, which may be called somatic and
reproductive—the cells of the body as opposed to those which are
concerned with reproduction. This differentiation was not at first
absolute, and indeed it is not always so to-day. Among the lower
Metazoa, such as the polypes, the capacity for reproduction still
exists to such a degree in the somatic cells, that a small number of
them are able to give rise to a new organism,—in fact new individuals
are normally produced by means of so-called buds. Furthermore, it is
well known that many of the higher animals have retained considerable
powers of regeneration; the salamander can replace its lost tail or
foot, and the snail can reproduce its horns, eyes, etc.
As the complexity of the Metazoan body increased, the two groups of
cells became more sharply separated from each other. Very soon the
somatic cells surpassed the reproductive in number, and during this
increase they became more and more broken up by the principle of the
division of labour into sharply separated systems of tissues. As these
changes took place, the power of reproducing large parts of the
organism was lost, while the power of reproducing the whole individual
became concentrated in the reproductive cells alone.
But it does not therefore follow that the somatic cells were compelled
to lose the power of unlimited cell-production, although in accordance
with the law of heredity, they could only give rise to cells which
resembled themselves, and belonged to the same differentiated
histological system. But as the fact of normal death seems to teach us
that they have lost even this power, the causes of the loss must be
sought outside the organism, that is to say, in the external conditions
of life; and we have already seen that death can be very well explained
as a secondarily acquired adaptation. The reproductive cells cannot
lose the capacity for unlimited reproduction, or the species to which
they belong would suffer extinction. But the somatic cells have lost
this power to a gradually increasing extent, so that at length they
became restricted to a fixed, though perhaps very large number of
cell-generations. This restriction, which implies the continual influx
of new individuals, has been explained above as a result of the
impossibility of entirely protecting the individual from accidents, and
from the deterioration which follows them. Normal death could not take
place among unicellular organisms, because the individual and the
reproductive cell are one and the same: on the other hand, normal death
is possible, and as we see, has made its appearance, among
multicellular organisms in which the somatic and reproductive cells are
distinct.
I have endeavoured to explain death as the result of restriction in the
powers of reproduction possessed by the somatic cells, and I have
suggested that such restriction may conceivably follow from a
limitation in the number of cell-generations possible for the cells of
each organ and tissue. I am unable to indicate the molecular and
chemical properties of the cell upon which the duration of its power of
reproduction depends: to ask this is to demand an explanation of the
nature of heredity—a problem the solution of which may still occupy
many generations of scientists. At present we can hardly venture to
propose any explanation of the real nature of heredity.
But the question must be answered as to whether the kind and degree of
reproductive power resides in the nature of the cell itself, or in any
way depends upon the quality of its nutriment.
Virchow, in his ‘Cellular Pathology,’ has remarked that the cells are
not only nourished, but that they actively supply themselves with food.
If therefore the internal condition of the cell decides whether it
shall accept or reject the nutriment which is offered, it becomes
conceivable that all cells may possess the power of refusing to absorb
nutriment, and therefore of ceasing to undergo further division.
Modern embryology affords us many proofs, in the segmentation of the
ovum, and in the subsequent developmental changes, that the causes of
the different forms of reproductive activity witnessed in cells lie in
the essential nature of the cells themselves. Why does the segmentation
of one half of certain eggs proceed twice as rapidly as that of the
other half? why do the cells of the ectoderm divide so much more
quickly than those of the endoderm? Why does not only the rate, but
also the number of cells produced (so far as we can follow them) always
remain the same? Why does the multiplication of cells in every part of
the blastoderm take place with the exact amount of energy and rapidity
necessary to produce the various elevations, folds, invaginations,
etc., in which the different organs and tissues have their origin, and
from which finally the organism itself arises? There can be no doubt
that the causes of all these phenomena lie within the cells themselves;
that in the ovum and the cells which are immediately derived from it,
there exists a tendency towards a certain determined (I might almost
say specific) mode and energy of cell-multiplication. And why should we
regard this inherited tendency as confined to the building up of the
embryo? why should it not also exist in the young, and later in the
mature animal? The phenomena of heredity which make their appearance
even in old age afford us proofs that a tendency towards a certain mode
of cell-multiplication continues to regulate the growth of the organism
during the whole of its life.
The above-mentioned considerations show us that the degree of
reproductive activity present in the tissues is regulated by internal
causes while the natural death of an organism is the termination—the
hereditary limitation—of the process of cell-division, which began in
the segmentation of the ovum.
Allow me to suggest a further consideration which may be compared with
the former. The organism is not only limited in time, but also in
space: it not only lives for a limited period, but it can only attain a
limited size. Many animals grow to their full size long before their
natural end: and although many fishes, reptiles, and lower animals are
said to grow during the whole of their life, we do not mean by this
that they possess the power of unlimited growth any more than that of
unlimited life. There is everywhere a maximum size, which, as far as
our experience goes, is never surpassed. The mosquito never reaches the
size of an elephant, nor the elephant that of a whale.
Upon what does this depend? Is there any external obstacle to growth?
Or is the limitation entirely imposed from within?