sedentary mode of life, the need for locomotion and for obtaining food
in fresh places may have arisen at some period of life. But whenever
formation of swarm-spores takes place instead of simple fission, this
does not depend upon the persistence of an ancestral form in the
ontogenetic cycle, but is due to the intercalation of an entirely new
ontogenetic stage, which happens to resemble an ancestral form, in the
possession of cilia, etc.
I imagine that I have now sufficiently explained the above proposition,
that the repetition of the phylogeny in the ontogeny does not and
cannot occur among unicellular organisms.
With the Polyplastides the opposite is the case. There is no species,
as far as we know, which does not—either in each individual, or after
long cycles which comprise many individuals (alternation of
generations)—invariably revert to the Monoplastid state. This applies
from the lowest forms, such as _Magosphaera_ and the Orthonectides, up
to the very highest. In the latter a great number of intermediate
phyletic stages always occur, although some have been omitted as the
result of concentration in the ontogeny, while others have sometimes
been intercalated.
Sexual reproduction is the obvious cause of this very important
arrangement. Even if this is an hypothesis rather than a fact we must
nevertheless accept it unconditionally, because it is a method of
reproduction found everywhere. It is the rule in every group of the
animal kingdom, and is only absent in a few species in which it is
replaced by parthenogenesis. In these latter instances sexual
reproduction may be local, and entirely absent in certain districts
only (_Apus_), or it may be only apparently wanting; in some cases
where it is undoubtedly absent, it is equally certain that it was
present at an earlier period (_Limnadia Hermanni_). We cannot as yet
determine whether its loss will not involve the degeneration and
ultimate extinction of the species in question.
If the essential nature of sexual reproduction depends upon the
conjugation of two equivalent but dissimilar morphological elements,
then we can understand that a multicellular being can only attain
sexual reproduction when a unicellular stage is present in its
development; for the coalescence of entire multicellular organisms in
such a manner that fusion would only take place between equivalent
cells, would seem to be impracticable. In the necessity for sexual
reproduction, there is therefore also implied the necessity for
reverting to the original condition of the Polyplastides—that of a
single cell—and upon this alone depends the fundamental law of
biogenesis. This law is therefore confined to the Polyplastides, and
does not apply to the Monoplastides; and Götte’s suggestion that the
latter fall back into the primitive condition of the organism during
their encystment (rejuvenescence), finds no support in this aspect of
the question.
I have on a previous occasion[91] referred the utility of death to the
ultimate fact that the unending life of the Metazoan body would be a
useless luxury, and to the fact that the individuals would necessarily
become injured in the course of time, and would be therefore ‘not only
valueless to the species, but ... even harmful, for they take the place
of those which are sound’ (l. c., p. 24). I might also have said that
such damaged individuals would sooner or later fall victims to some
accidental death, so that there would be no possibility of real
immortality. I now propose to examine this statement a little more
closely, and to return to a question which has already been alluded to
before.
It is obvious that the advantages above set forth did not form the
motive which impelled natural selection to convert the immortal life of
the Monoplastides into the life of limited duration possessed by the
Heteroplastides, or more correctly, which led to the restriction of
potential immortality to the reproductive cells of the latter. It is at
any rate theoretically conceivable that a struggle might arise between
the mortal and immortal individuals of a certain Metazoan species, and
that natural selection might secure the success of the former, because
the longer the immortal individuals lived, the more defective they
became, and as a result gave rise to weaker offspring in diminished
numbers. Probably no one would be bold enough to suggest such a crude
example of natural selection. And yet I venture to think that the
principle of natural selection is here also to be taken into account,
and even plays, although in a negative rather than a positive way, a
very essential part in determining the duration of life in the Metazoa.
When the somatic cells of the first Heteroplastides ceased to be
immortal, such a loss would not in any way have precluded them from
regaining this condition. Just as, with the differentiation of the
first somatic cells of the lowest Heteroplastides, their duration was
limited to that of a single cell-generation,—so it must have been
possible for them, at a later period and if the necessity arose, to
lengthen their duration over two, three, or more generations. And if my
theory of the duration of life in the Metazoa is well founded, these
cells have as a matter of fact increased their duration, to an extent
about equal to that of the organism to which they belong. There is no
ground whatever for the assumption that it is impossible to fix the
number of cell-generations at infinity,—as actually happens in the case
of the reproductive cells,—but on the other hand it has already been
shown to be obvious that such an extension is opposed to the principle
of utility. It could never be to the advantage of a species to produce
crippled individuals, and therefore the infinite duration of
individuals has never reappeared among the Metazoa. So far the limited
duration of Metazoan life may be attributed to the worthlessness or
even the injurious nature of individuals, which although immortal, were
nevertheless liable to wear and tear. This fact explains why
immortality has never reappeared, it explains the predominance of
death, but it was not the single primary cause of this phenomenon. The
perishable and vulnerable nature of the _soma_ was the reason why
nature made no effort to endow this part of the individual with a life
of unlimited length.
Götte considers that death is inherent in reproduction, and in a
certain sense this is true, but not in the general way supposed by him.
I have endeavoured to show above that it is most advantageous for the
preservation of the species among the lowest Metazoa, that the body
should consist of a relatively small number of cells, and that the
reproductive cells should ripen simultaneously and all escape together.
If this conclusion be accepted, the uselessness of a prolonged life to
the somatic cells is obvious, and the occurrence of death at the time
of the extrusion of the reproductive cells is explained. In this manner
death (of the _soma_) and reproduction are here made to coincide.
This relation of reproduction to death still exists in a great number
of the higher animals. But such an association, together with the
simultaneous ripening of the reproductive cells, has not been
maintained continuously in the past. As the _soma_ becomes larger and
more highly organized, it is able to withstand more injuries, and its
average duration of life will extend: _pari passu_ with these changes
it will become increasingly advantageous not only for the number of
reproductive cells to be multiplied, but also for the time during which
they are produced to be prolonged. In this manner a lengthening of the
reproductive period arises, at first continuously and then
periodically. It is beyond my present purpose to consider in detail the
conditions upon which this lengthening depends, but I would emphasize
the fact that a lengthening of life is connected with the increase in
the duration of reproduction, while on the other hand there is no
reason to expect life to be prolonged beyond the reproductive period;
so that the end of this period is usually more or less coincident with
death.
A further prolongation of life could only take place when the parent
begins to undertake the duty of rearing the young. The most primitive
form of this is found among those animals, which do not expel their
reproductive cells as soon as they are ripe but retain them within
their bodies, so that the early stages of development take place under
the shelter of the parent organism. Associated with such a process
there is frequently a necessity for the germs to reach a certain spot,
where alone their further development can take place. Thus a segment of
a tapeworm lives until it has brought the embryos into a position which
affords the possibility of their passive transference to the stomach of
their special host. But the duration of life is first materially
lengthened when the offspring begin to be really tended, and as a
general rule the increase in length is exactly proportional to the time
which is demanded by the care of the young. Accurately conducted
observations are wanting upon this precise point, but the general
tendency of the facts, as a whole, cannot be doubted. Those insects of
which the care for their offspring terminates with the deposition of
eggs at the appropriate time, place, etc., do not survive this act; and
the duration of life in such imagos is shorter or longer according as
the eggs are laid simultaneously or ripen gradually. On the other hand,
insects—such as bees and ants—which tend their young, have a life which
is prolonged for years.
But the lengthening of the reproductive period alone may result in a
marked increase in the length of life, as is proved by the queen-bee.
In all these cases it is easy to imagine the operation of natural
selection in producing such alterations in the duration of life, and
indeed we might accurately calculate the amount of increase which would
be produced in any given case if the necessary data were available,
viz. the physiological strength of the body, and its relations to the
external world, such as, for instance, the power of obtaining food at
various periods of life, the expenditure of energy necessary for this
end, and the statistics of destruction, that is, the probabilities in
favour of the accidental death of a single individual at any given
time. These statistics must be known both for the imagos, larvae, and
eggs; for the lower they are for the imagos, and the higher for the
larvae and eggs, the more advantageous will it be, _ceteris paribus_,
for the number of eggs produced by the imago to be increased, and the
more probable it would therefore be that a long reproductive period,
involving a lengthening of the life of the imago, would be introduced.
But we are still far from being able to apply mathematics to the
phenomena of life; the factors are too numerous, and no attempt has
been made as yet to determine them with accuracy.
But we must at least admit the principle that both the lengthening and
shortening of life are possible by means of natural selection, and that
this process is alone able to render intelligible the exact adaptation
of the length of life to the conditions of existence.
A shortening of the normal duration of life is also possible; this is
shown in every case of sudden death, after the deposition of the whole
of the eggs at a single time. This occurs among certain insects, while
nearly allied forms of which the oviposition lasts over many days
therefore possess a correspondingly long imago-life. The _Ephemeridae_
and Lepidoptera afford many examples of this, and in an earlier work I
have collected some of them[92]. The humming-bird hawk-moth flies about
for weeks laying an egg here and there, and, like the allied poplar
hawk-moth and lime hawk-moth, probably dies when it has deposited all
the eggs which can be matured with the amount of nutriment at its
disposal. Many other Lepidoptera, such as the majority of butterflies,
fly about for weeks depositing their eggs, but others, such as the
emperor-moths and lappet-moths, lay their eggs one after another and
then die. The eggs of the parthenogenetic _Psychidae_ are laid directly
after the imago has left the cocoon, and death ensues immediately, so
that the whole life of the imago only lasts for a few hours. No one
could look upon this brief life as a primitive arrangement among
Lepidoptera, any more than we do upon the absence of wings in the
female _Psychidae_; shortening of life here is therefore clearly
explicable.
In such cases have we any right to speak of the fatal effect of
reproduction? We may certainly say that these insects die of
exhaustion; their vital strength is used up in the last effort of
laying eggs, and in the case of the males, in the act of copulation.
Reproduction is here certainly the most apparent cause of death, but a
more remote and deeper cause is to be found in the limitation of vital
strength to the length and the necessary duties of the reproductive
period. The fact that there are female Lepidoptera which, like the
emperor-moths, do not feed in the imago-state, proves the truth of this
statement. They still possess a mouth and a complete alimentary canal,
but they have no spiral ‘tongue,’ and do not take food of any kind, not
even a drop of water. They live in a torpid condition for days or weeks
until fertilization is accomplished, and then they lay their eggs and
die. The habit of extracting honey from flowers—common to most
hawk-moths and butterflies—would not have thus fallen into disuse, if
the store of nutriment, accumulated in the form of the fat-bodies,
during the life of the caterpillar, had not been exactly sufficient to
maintain life until the completion of oviposition. The fact that the
habit of taking food has been thus abandoned is a proof that the
duration of life beyond the reproductive period would not be to the
advantage of the species.
The protraction of existence into old age among the higher Metazoa
proves that death is not a necessary consequence of reproduction. It
seems to me that Götte’s statement ‘that the appearances of senility
must not be regarded as the general cause of death’ is not in
opposition to my opinions but rather to those which receive general
acceptance. I have myself pointed out that ‘death is not always
preceded by senility or a period of old age[93].’
The materials are wanting for a comprehensive investigation of the
causes which first introduced this period among the higher Metazoa; in
fact the most fundamental data are absent, for we do not even know the
part of the animal kingdom in which it first appeared: we cannot even
state the amount by which the duration of life exceeds that of the
period of reproduction, or what is the value to the species of this
last stage in the life of the individual.
It is in these general directions that we must seek for the
significance of old age. It is obviously of use to man, for it enables
the old to care for their children, and is also advantageous in
enabling the older individuals to participate in human affairs and to
exercise an influence upon the advancement of intellectual powers, and
thus to influence indirectly the maintenance of the race. But as soon
as we descend a step lower, if only as far as the apes, accurate facts
are wanting, for we are, and shall probably long be, ignorant of the
total duration of their life, and the point at which the period of
reproduction ceases.
* * * * *
I must here break off in the midst of these considerations, rather than
conclude them, for much still remains to be said. I hope, nevertheless,
that I have thrown new light upon some important points, and I now
propose to conclude with the following short abstract of the results of
my enquiry.
I. Natural death occurs only among multicellular beings; it is not
found among unicellular organisms. The process of encystment in the
latter is in no way comparable with death.
II. Natural death first appears among the lowest Heteroplastid Metazoa,
in the limitation of all the cells collectively to one generation, and
of the somatic or body-cells proper to a restricted period: the somatic
cells afterwards in the higher Metazoa came to last several and even
many generations, and life was lengthened to a corresponding degree.
III. This limitation went hand in hand with a differentiation of the
cells of the organism into reproductive and somatic cells, in
accordance with the principle of division of labour. This
differentiation took place by the operation of natural selection.
IV. The fundamental biogenetic law applies only to multicellular
beings; it does not apply to unicellular forms of life. This depends on
the one hand upon the mode of reproduction by fission which obtains
among the Monoplastides (unicellular organisms), and on the other upon
the necessity, induced by sexual reproduction, for the maintenance of a
unicellular stage in the development of the Polyplastides
(multicellular organisms).
V. Death itself, and the longer or shorter duration of life, both
depend entirely on adaptation. Death is not an essential attribute of
living matter; it is neither necessarily associated with reproduction,
nor a necessary consequence of it.
* * * * *
In conclusion, I should wish to call attention to an idea which is
rather implied than expressed in this essay:—it is, that reproduction
did not first make its appearance coincidently with death. Reproduction
is in truth an essential attribute of living matter, just as is the
growth which gives rise to it. It is as impossible to imagine life
enduring without reproduction as it would be to conceive life lasting
without the capacity for absorption of food and without the power of
metabolism. Life is continuous and not periodically interrupted: ever
since its first appearance upon the earth, in the lowest organisms, it
has continued without break; the forms in which it is manifested have
alone undergone change. Every individual alive to-day—even the very
highest—is to be derived in an unbroken line from the first and lowest
forms.
------------------------------------------------------------------------
Footnotes for Chapter III.
Footnote 59:
‘Ueber den Ursprung des Todes,’ Hamburg and Leipzig, 1883.
Footnote 60:
As in the case of the bodies of monks on the Great St. Bernard, or
the dried-up bodies in the well-known Capuchine Monastery at Palermo.
Footnote 61:
Professor Gruber informs me that among the Infusoria of the harbour
of Genoa, he has observed a species which encysts upon one of the
free-swimming Copepoda. He has often found as many as ten cysts upon
one of these Copepods, and has observed the escape of their contents
whenever the water under the cover-glass began to putrefy. Here
advantage is probably gained in the rapid transport of the cyst by
the Crustacean.
Footnote 62:
The views of most biologists who have worked at this subject agree in
all essentials with that expressed above. Bütschli says (Bronn’s
‘Klassen und Ordnungen des Thierreichs,’ Protozoa, p. 148): ‘The
process of encystment does not appear to have originally borne any
direct relation to reproduction: it appears on the contrary to have
taken place originally,—as it frequently does at the present
day,—either for the protection of the organism against injurious
external influences, such as desiccation or the fatal effects of
impure water, etc.; and also to enable the organism, after taking up
an unusually abundant supply of food, to assimilate it in safety.’
Balbiani (‘Journ. de Micrographie,’ Tom. V. 1881, p. 293) says in
reference to the Infusoria, ‘Un petit nombre d’espèces, au lieu de se
multiplier à l’état de vie active, se reproduisent dans une sorte
d’état de repos, dit état d’enkystement. Ces sortes de kystes peuvent
être désignés sous le nom de kystes de reproduction, par opposition
avec d’autres kystes, dans lesquels les Infusoires se renferment pour
se soustraire à des conditions devenues défavorables du milieu qu’ils
habitent, le manque d’air, le dessèchement, etc.—ceux-ci sont des
kystes de conservation....’
Footnote 63:
This is of importance in so far as single individuals might be thus
compelled to encyst even when the existing external conditions of
life do not require it. The substance which _Actinosphaerium_, for
example, employs in the secretion of its thick siliceous cyst must
have been gradually accumulated by means of a process peculiar to the
species. We can scarcely be in error if we assume that the silica
accumulated in the organism cannot increase to an unlimited extent
without injury to the other vital processes and that the secretion of
the cyst must take place as soon as the accumulation has exceeded a
certain limit. Thus we can understand that encystment may occur
without any external necessity. Similarly, certain Entomostraca (_e.
g._ _Moina_) produce winter-eggs in a particular generation, and
these are formed even when the animals are kept in a room protected
from cold and desiccation.
Footnote 64:
Upon this point Professor Gruber intends to publish an elaborate
memoir.
Footnote 65:
This view has not even been proved for _Actinosphaerium_, upon which
Götte chiefly relies. The observations which we now possess merely
indicate that the animal contracts to the smallest volume possible.
Compare F. E. Schulze, ‘Rhizopodenstudien,’ I, Arch. f. mikr. Anat.
Bd. 10, p. 328; and Karl Brandt, ‘Ueber Actinosphaerium Eichhornii,’
Inaug. Diss.; Halle, 1877.
Footnote 66:
The conception of Protozoa and Metazoa does not correspond exactly
with that of unicellular and multicellular beings, for which Götte
has proposed the names Mono- and Polyplastides.
Footnote 67:
Among the Rhizopoda encystment is only known in fresh-water forms,
and not in a single one of the far more numerous marine forms which
possess shells (see Bütschli, ‘Protozoa,’ p. 148); the marine
Rhizopoda are not exposed to the effects of desiccation or frost, and
thus the strongest motives for the process of encystment do not
exist, at least among forms possessing a shell.
Footnote 68:
I trust that it will not be objected that the germ-cells cannot be
immortal, because they frequently perish in large numbers, as a
result of the natural death of the individual. There are certain
definite conditions under which alone a germ-cell can render its
potential immortality actual, and these conditions are for the most
part fulfilled with difficulty (fertilization, etc.). It follows from
this fact that the germ-cells must always be produced in numbers
which reach some very high multiple of the necessary number of
offspring, if these latter are to be ensured for the species. If in
the natural death of the individual the germ-cells must also die, the
_natural_ death of the _soma_ becomes a cause of _accidental_ death
to the germ-cells.
Footnote 69:
l. c., p. 78.
Footnote 70:
l. c., p. 47.
Footnote 71:
‘Entwicklungsgeschichte der Unke,’ Leipzig, 1875, p. 65.
Footnote 72:
Id., p. 842.
Footnote 73:
‘Ursprung des Todes,’ p. 79.
Footnote 74:
l. c., p. 42.
Footnote 75:
‘Contributions à l’histoire des Mesozoaires. Recherches sur
l’organisation et le développement embryonnaire des Orthonectides,’
Arch. de Biologie, vol. iii. 1882.
Footnote 76:
l. c., p. 37.
Footnote 77:
Julin does not enter into further details on this point, and it is
not quite clear at what precise time the cells of the ectoderm
atrophy; but this is irrelevant to the origin of death, since the
granular mass surrounding the egg-cells at any rate belongs to the
_soma_ of the mother.
Footnote 78: