explain the transmission of the cause of death—which first appeared in
the Orthonectides—from these organisms to all later Metazoa, until the
very highest forms are reached. Exact proofs of this supposition are
unfortunately wanting, and the evidence is confined to the collection
of a number of cases in which death and reproduction take place nearly
or quite simultaneously. These would prove nothing, even if _post hoc_
were always _propter hoc_; and there are, opposed to them, a number of
cases in which reproduction and death take place at different times. In
obtaining evidence for ‘the fatal influence of reproduction,’ is it
possible to point to every case of sudden death after the act of
oviposition or fertilization? These cases occur among many of the
higher animals, especially in Insects, and were collected by me in an
earlier work[79]. It is obvious that such cases are exceptional, but in
a restricted sense it is quite true, as far as these individual
instances are concerned, that death appears as a consequence of
reproduction. The male bee, which invariably dies while pairing, is
undoubtedly killed in consequence of a very powerful nervous shock; and
the female Psychid, which has laid all her eggs at once, dies of
‘exhaustion’—however we may attempt to explain the term on
physiological principles.
Can we conclude from these cases that the effects of reproduction are,
in Götte’s sense, universally fatal; that reproduction is the positive
and ‘exclusive explanation of natural death’? (l. c., p. 32.) I need
not linger over these isolated examples, but I turn at once to the
foundation of the whole conclusion—a foundation which is obviously
unable to support the superstructure erected on it. Götte formally
derives the idea that death is a necessary condition of reproduction,
from a very heterogeneous collection of facts. When we examine this
collection we find that the process which is taken to be death is not
the same thing in all these instances, while the same is true of the
influence of reproduction by which death is supposed to be caused. The
whole conception arises out of the process of encystment, which is
regarded as the building-up of reproductive material—that is, as true
reproduction; and since, according to Götte’s view, the formation of
germs is always intimately connected with an arrest of life, and since,
by his own definition, this stand-still of life is equivalent to death,
it follows that, with such a theory, reproduction, in its essential
nature, must be inseparably connected with death. It is necessary at
this juncture to remember what Götte means by the process of
rejuvenescence, and to point out that he is dealing with something
quite different from ‘the fatal influence of reproduction,’ which was
just now mentioned with regard to insects. ‘Rejuvenescence,’ bound up
as it is with encystment and reproduction, is, according to Götte, ‘a
re-coining of the specific protoplasm, by means of which the identity
of its substance is fixed by heredity,’ a ‘marvellous process in which
phenomena the most important in the whole life of the animal, and in
fact of all organisms—reproduction and death—have their roots’ (l. c.,
p. 81). Whether such re-coining really takes place or not, at any rate
I claim to have shown above that it does not correspond with death in
the Metazoa, and—if it is represented at all in these latter—that it
ought to be looked for in the reproductive cells; and indeed, in
another passage, Götte himself has placed the process in these cells.
While, among the Monoplastids, according to Götte, the causes of the
supposed death lie hidden in this mysterious change of the organism
into reproductive material, Götte asserts that among the Polyplastids
(such as _Magosphaera_, hypothetically perfected so as to form a
genuine Polyplastid), the causes of death operate so that the organism
breaks up into its component cells, all these being still reproductive
cells—a process which, unlike ‘rejuvenescence,’ has nothing mysterious
about it, and which is certainly not genuine death. In the
Orthonectid-like animals death does not occur as a consequence of the
dispersal of the reproductive cells, but rather because the part of the
animal which remains is so small and effete that, being unable to
support itself, it necessarily dies. From this point at least the
object of death and the conception of it remain the same, but now the
idea of reproduction undergoes a change. When the Rhabdite females of
_Ascaris_ are eaten up by their offspring, is this mode of death
connected with the ‘rejuvenescence of protoplasm’? (l. c., p. 34.) Is
there any deep underlying relationship between such an end and the
essential nature of reproduction? The same question may be asked with
regard to the ‘Redia or the Sporocyst of Trematodes, which are
converted into slowly dying sacs during the growth of the Cercariae
within them.’ We cannot speak of the ‘fatal influence of reproduction’
among tape-worms just because ‘in the ripe segments the whole
organization degenerates under the influence of the excessive growth of
the uterus.’ It certainly degenerates, but only so far as the
developing mass of eggs demands. In fact, at a sufficiently high
temperature, death does not occur, and such mature segments of
tape-worms creep about of their own accord. We cannot fail to recognize
that in this as well as in the above-mentioned cases we have to do with
adaptation to certain very special conditions of existence—an
adaptation leading to an immense development of reproductive cells in a
mother organism which can no longer take in nourishment, or which has
become entirely superfluous because its duty to its species is already
fulfilled. If this is an example of death inherent in the essential
nature of reproduction, then so is the death of a mature segment of a
tapeworm in the gastric juices of the pig that eats it.
With Götte, the conception of reproduction, like the conception of
death, is a protean form, which he welcomes in any shape, if only he
can use it as evidence. If death is a necessary consequence of
reproduction, its cause must be always essentially the same, and might
be expressed in one of the following suggestions:—(1) in the necessity
for a ‘re-coining’ of the protoplasm of the germ-cells; but here death
could only affect the germ-cells themselves: (2) perhaps in the
withdrawal of nourishment by the mass of developing reproductive
material, just as death occurs sometimes among men by the excessive
drain on the system caused by morbid tumours: (3) or in consequence of
the development of the offspring in the body of the mother; this
however would only affect the females, and could therefore have no deep
and general significance: (4) from the extrusion of the sexual
cells,—ova or spermatozoa,—and in the impossibility of further
nourishment which is consequent upon this extrusion—(Orthonectides?):
or (5) finally in an excessively powerful nervous shock brought about
by the ejection of the reproductive cells.
But no one of these alternatives is the universal and inevitable cause
of death. This proves irrefutably that death does not proceed as an
intrinsic necessity from reproduction, although it may be connected
with the latter, sometimes in one way and sometimes in another. But we
must not overlook the fact that in many cases death is not connected
with reproduction at all; for many Metazoa survive for a longer or
shorter period after the reproductive processes have ceased.
In fact, I believe I have definitely shown that no process exists among
unicellular animals which is at all comparable with the natural death
of the higher organisms. Natural death first appeared among
multicellular beings, and among these first in the Heteroplastids.
Furthermore, it was not introduced from any absolute intrinsic
necessity inherent in the nature of living matter, but on grounds of
utility, that is from necessities which sprang up, not from the general
conditions of life, but from those special conditions which dominate
the life of multicellular organisms. If this were not so, unicellular
beings must also have been endowed with natural death. I have already
expressed these ideas elsewhere[80], and have briefly indicated how
far, in my opinion, natural death is expedient for multicellular
organisms. I found the essential reason for confining the life of the
Metazoa to a fixed and limited period, in the wear and tear to which an
individual is exposed in the course of a life-time. For this reason,
‘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’ (l. c., p. 24). Death seemed to me to be expedient since
‘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’ (l.
c., p. 24).
I still adhere entirely to this explanation; not of course in the sense
that an actual physical struggle has ever taken place between the
mortal and immortal varieties of any species. If Götte understood me
thus, he may be justified by the brief explanations given in the essay
to which I have alluded; but when he also attributes to me the opinion
that such hypothetically immortal Metazoa had but a very limited period
for reproduction, I fail to see what part of the essay in question can
be brought forward in support of his statement. Only under some such
supposition can I be reproached with having assumed the existence of a
process of natural selection which could never be effective, because
any advantage which accrued to the species from the shortening of the
duration of life could not make itself felt in a more rapid propagation
of the short-lived individuals. The statement ‘that in this and in
every other case it is a sufficient explanation of the processes of
natural selection to render it probable that any kind of advantage is
gained’[81] is indeed erroneous. The explanation ought rather to be
‘that the forms in question would for ever transmit their characters to
a greater number of descendants than the other forms.’ I have not
however as yet attempted to think out in detail such processes of
natural selection as would limit the somatic part of the Metazoan body
to a short term of existence, and I only wished to emphasize the
general principle lying at the basis of the whole process, without
stating the precise manner in which it operates.
If I now attempt to take this course, and to reconstruct theoretically
the gradual appearance of natural death in the Metazoa, I must begin by
again alluding to Götte’s criticisms in reference to the operation of
natural selection.
I consider death as an adaptation, and believe that it has arisen by
the operation of natural selection. Götte[82], however, concludes from
this that ‘the first origin of hereditary and consequently (for the
organization in question) necessary death, is not explained but already
assumed.’ ‘The operation and significance of the principle of utility
consists in selecting the fittest from among the structures and
processes which are at hand, and not in directly creating new ones.
Every new structure arises at first, quite independently of any
utility, from certain material causes present in a number of
individuals, and when it has proved useful and is transmitted, it
extends, according to the laws of the survival of the fittest, in the
group of animals in which it appeared. This extension will undergo
further increase with every advance in utility which results from
further structural changes, until it extends over the whole group. So
that usefulness effects the preservation and the distribution of new
structures, but has nothing whatever to do with the causes of their
primary origin and their consequent transmission to all other
individuals. Indeed, on these hereditary causes the necessity of the
structures in question depends, so that their usefulness in no way
explains their necessity.’
‘These conclusions, when applied to the origin of natural death called
forth by internal causes, would show that it became inevitable and
hereditary in a number of the originally immortal Metazoa, before there
could be any question as to the benefits derived from its influence.
Such influence must have consisted in the fact that more descendants
survived the struggle for existence and were able to enter upon
reproduction among the individuals which had inherited the
predisposition to die than among the potentially immortal beings which
would be damaged in the struggle for existence, and would therefore be
exposed to still further injuries. The existing necessity for natural
death in all Metazoa might therefore be derived in an unbroken line of
descent from the first mortal Metazoan, of which the death became
inevitable from internal causes, before the principle of utility could
operate in favour of its dissemination.’
In reply to this I would urge: that it has been very often maintained
that natural selection can produce nothing new, but can only bring to
the front something which existed previously to the exercise of choice;
but this argument is only true in a very limited sense. The complex
world of plants and animals which we see around us contains much that
we should call new in comparison with the primitive beings from which,
as we believe, everything has developed by means of natural selection.
No leaves or flowers, no digestive system, no lungs, legs, wings, bones
or muscles were present in the primitive forms, and all these must have
arisen from them according to the principle of natural selection. These
primitive forms were in a certain sense predestined to develope them,
but only as possibilities, and not of necessity; nor were they
preformed in them. The course of development, as it actually took
place, first became a necessity by the action of natural selection,
that is by the choice of various possibilities, according to their
usefulness in fitting the organism for its external conditions of life.
If we once accept the principle of natural selection, then we must
admit that it really can create new structures, instincts, etc., not
suddenly or discontinuously, but working by the smallest stages upon
the variations that appear. These changes or variations must be looked
upon as very insignificant, and are, as I have of late attempted to
show[83], quantitative in their nature; and it is only by their
accumulation that changes arise which are sufficiently striking to
attract our attention, so that we call them ‘new’ organs, instincts,
etc.
These processes may be compared to a man on a journey who proceeds from
a certain point on foot by short stages, at any given time, and in any
direction. He has then the choice of an infinite number of routes over
the whole earth. If such a man begins his wanderings in obedience to
the impulse of his own will, his own pleasure or interest,—proceeding
forwards, to the right or left, or even backwards, with longer or
shorter pauses, and starting at any particular time,—it is obvious that
the route taken lies in the man himself and is determined by his own
peculiar temperament. His judgment, experience, and inclination will
influence his course at each turn of his journey, as new circumstances
arise. He will turn aside from a mountain which he considers too lofty
to be climbed; he will incline to the right, if this direction appears
to afford a better passage over a swollen stream; he will rest when he
reaches a pleasant halting-place, and will hurry on when he knows that
enemies beset him. And in spite of the perfectly free choice open to
him, the course he takes is in fact decided by both the place and time
of his starting and by circumstances which—always occurring at every
part of the journey—impel him one way or the other; and if all the
factors could be ascertained in the minutest detail, his course could
be predicted from the beginning.
Such a traveller represents a species, and his route corresponds with
the changes which are induced in it by natural selection. The changes
are determined by the physical nature of the species, and by the
conditions of life by which it is surrounded at any given time. A
number of different changes may occur at every point, but only that one
will actually develope which is the most useful, under existing
external conditions. The species will remain unaltered as long as it is
in perfect equilibrium with its surroundings, and as soon as this
equilibrium is disturbed it will commence to change. It may also happen
that, in spite of all the pressure of competing species, no further
change occurs because no one of the innumerable very slight changes,
which are alone possible at any one time, can help in the struggle;
just as the traveller who is followed by an overpowering enemy, is
compelled to succumb when he has been driven down to the sea. A boat
alone could save him, without it he must perish; and so it sometimes
happens that a species can only be saved from destruction by changes of
a conspicuous kind, and these it is unable to produce.
And just as the traveller, in the course of his life, can wander an
unlimited distance from his starting-point, and may take the most
tortuous and winding route, so the structure of the original organism
has undergone manifold changes during its terrestrial life. And just as
the traveller at first doubts whether he will ever get beyond the
immediate neighbourhood of his starting-point, and yet after some years
finds himself very far removed from it—so the insignificant changes
which distinguish the first set of generations of an organism lead on
through innumerable other sets, to forms which seem totally different
from the first, but which have descended from them by the most gradual
transition. All this is so obvious that there is hardly any need of a
metaphor to explain it, and yet it is frequently misunderstood, as
shown by the assertion that natural selection can create nothing new:
the fact being that it so adds up and combines the insignificant small
deviations presented by natural variation, that it is continually
producing something new.
If we consider the introduction of natural death in connection with the
foregoing statements, we may imagine the process as taking place in
such a way that,—with the differentiation of Heteroplastids from
Homoplastids, and the appearance of division of labour among the
homogeneous cell-colonies,—natural selection not only operated upon the
physiological peculiarities of feeding, moving, feeling, or
reproduction, but also upon the duration of the life of single cells.
At this developmental stage there would, at any rate, be no further
necessity for maintaining the power of limitless duration. The somatic
cells might therefore assume a constitution which excluded the
possibility of unending life, provided only that such a constitution
was advantageous for them.
It may be objected that cells of which the ancestors possessed the
power of living for ever, could not become potentially mortal (that is
subject to death from internal causes) either suddenly or gradually,
for such a change would contradict the supposition which attributes
immortality to their ancestors and to the products of their division.
This argument is valid, but it only applies so long as the descendants
retain the original constitution. But as soon as the two products of
the fission of a potentially immortal cell acquire different
constitutions by unequal fission, another possibility arises. Now it is
conceivable that one of the products of fission might preserve the
physical constitution necessary for immortality, but not the other;
just as it is conceivable that such a cell—adapted for unending
life—might bud off a small part, which would live a long time without
the full capabilities of life possessed by the parent cell; again, it
is possible that such a cell might extrude a certain amount of organic
matter (a true excretion) which is already dead at the moment it leaves
the body. Thus it is possible that true unequal cell-division, in which
only one half possesses the condition necessary for increasing, may
take place; and in the same way it is conceivable that the constitution
of a cell determines the fixed duration of its life, examples of which
are before us in the great number of cells in the higher Metazoa, which
are destroyed by their functions. The more specialized a cell becomes,
or in other words, the more it is intrusted with only one distinct
function, the more likely is this to be the case: who then can tell us,
whether the limited duration of life was brought about in consequence
of the restricted functions of the cell or whether it was determined by
other advantages[84]? In either case we must maintain that the
disadvantages arising from a limited duration of the cells are more
than compensated for by the advantages which result from their highly
effective specialized functions. Although no one of the functions of
the body is necessarily attended by the limited duration of the cells
which perform it, as is proved by the persistence of unicellular forms,
yet any or all of them might lead to such a limitation of existence
without in any way injuring the species, as is proved by the Metazoa.
But the reproductive cells cannot be limited in this way, and they
alone are free from it. They could not lose their immortality, if
indeed the Metazoa are derived from the immortal Protozoa, for from the
very nature of that immortality it cannot be lost. From this point of
view the body, or _soma_, appears in a certain sense as a secondary
appendage of the real bearer of life,—the reproductive cells.
Just as it was possible for the specific somatic cells to be
differentiated from among the chemico-physical variations which
presented themselves in the protoplasm, by means of natural selection,
until finally each function of the body was performed by its own
special kind of cell; so it might be possible for only those variations
to persist the constitution of which involved a cessation of activity
after a certain fixed time. If this became true of the whole mass of
somatic cells, we should then meet with natural death for the first
time. Whether we ought to regard this limitation of the life of the
specific somatic cells as a mere consequence of their differentiation,
or at the same time as a consequence of the powers of natural selection
especially directed to such an end,—appears doubtful. But I am myself
rather inclined to take the latter view, for if it was advantageous to
the somatic cells to preserve the unending life of their ancestors—the
unicellular organisms, this end might have been achieved, just as it
was possible at a later period, in the higher Metazoa, to prolong both
the duration of life and of reproduction a hundred- or a thousand-fold.
At any rate, no reason can be given which would demonstrate the
impossibility of such an achievement.
With our inadequate knowledge it is difficult to surmise the immediate
causes of such a selective process. Who can point out with any feeling
of confidence, the direct advantages in which somatic cells, capable of
limited duration, excelled those capable of eternal duration? Perhaps
it was in a better performance of their special physiological tasks,
perhaps in additional material and energy available for the
reproductive cells as a result of this renunciation of the somatic
cells; or perhaps such additional power conferred upon the whole
organism a greater power of resistance in the struggle for existence,
than it would have had, if it had been necessary to regulate all the
cells to a corresponding duration.
But we are not at present able to obtain a clear conception of the
internal conditions of the organism, especially when we are dealing
with the lowest Metazoa, which seem to be very rarely found at the
present day, and of which the vital phenomena we only know as they are
exhibited by two species, both of doubtful origin. Both species have
furthermore lost much of their original nature, both in structure and
function, as a result of their parasitic mode of life. Of the
Orthonectides and Dicyemidae we know something, but of the reproduction
in the single free non-parasitic form, discovered by F. E. Schulze and
named by him _Trichoplax adhaerens_, we know nothing whatever, and of
its vital phenomena too little to be of any value for the purpose of
this essay.