death in the Metazoa from the Orthonectides, as Götte endeavoured to
derive it, when he overlooked the fact that, according to his theory,
natural death is inherited from the Monoplastids and cannot therefore
have arisen anew in the Polyplastids. According to this theory, death
must necessarily have appeared in the lowest Metazoa as a result of the
extrusion of the germ-cells, and by continual repetition must have
become hereditary. We must not however forget that, in this case, the
cause of death is exclusively external, by which I mean that the
somatic cells which remained after the extrusion of the reproductive
cells, were unable to feed any longer or at any rate to an adequate
extent; and that the cause of their death did not lie in their
constitution, but in the unfavourable conditions which surrounded them.
This is not so much a process of natural death as of artificial death,
regularly appearing in each individual at a corresponding period,
because, at a certain time of life, the organism becomes influenced by
the same unfavourable conditions. It is just as if the conditions of
life invariably led to death by starvation at a certain stage in the
life of a certain species. But we know that death arises from purely
internal causes among the higher Metazoa, and that it is anticipated by
the whole organisation as the normal end of life. Hence nothing is
gained by this explanation founded on the Orthonectides, and we should
have to seek further and in a later stage of the development of the
Metazoa, for the internal causes of true natural death.
Another theory might be based upon the supposition that natural death
has been derived, in the course of time, from an artificial death which
always appeared at the same stage of each individual life—as we have
supposed to be the case in the Orthonectides. I cannot agree with this
view, because it involves the transmission of acquired characters,
which is at present unproved and must not be assumed to occur until it
has been either directly or indirectly demonstrated[85]. I cannot
imagine any way in which the somatic cells could communicate this
assumed death by starvation to the reproductive cells in such a manner
that the somatic cells of the resulting offspring would spontaneously
die of hunger in the same manner and at a corresponding time as those
of the parent. It would be as impossible to imagine a theoretical
conception of such transmission as of the supposed instance of kittens
being born without a tail after the parent’s tail had been docked;
although to make the cases parallel the kittens’ tails ought to be lost
at the same period of life as that at which the parent lost hers. And
in my opinion we do not add to the intelligibility of such an idea by
assuming the artificial removal of tails through hundreds of
generations. Such changes, and indeed all changes, are, as I think,
only conceivable and indeed possible when they arise from within, that
is, when they arise from changes in the reproductive cells. But I find
no difficulty in believing that variations in these cells took place
during the transition from Homoplastids to Heteroplastids, variations
which formed the material upon which the unceasing process of natural
selection could operate, and thus led to the differentiation of the
previously identical cells of the colony into dissimilar ones—some
becoming perishable somatic cells, and others the immortal reproductive
cells.
It is at any rate a delusion to believe that we have explained natural
death, by deriving it from the starvation of the _soma_ of the
Orthonectides, by the aid of the unproved assumption of the
transmission of acquired variations. We must first explain why these
organisms produce only a limited number of reproductive cells which are
all extruded at once, so that the _soma_ is rendered helpless. Why
should not the reproductive cells ripen in succession as they do
indirectly among the Monoplastides, that is to say in a succession of
generations, and as they do directly in great numbers among the
Metazoa? There would then be no necessity for the _soma_ to die, for a
few reproductive cells would always be present, and render the
persistence of the individual possible. In fact, the whole
arrangement—the formation of reproductive cells at one time only, and
their sudden extrusion,—presupposes the mortality of the somatic cells,
and is an adaptation to it, just as this mortality itself must be
regarded as an adaptation to the simultaneous ripening and sudden
extrusion of the generative cells. In short, there is no alternative to
the supposition stated above, viz. that the mortality of the somatic
cells arose with the differentiation of the originally homogeneous
cells of the Polyplastids into the dissimilar cells of the
Heteroplastids. And this is the first beginning of natural death.
Probably at first the somatic cells were not more numerous than the
reproductive cells, and while this was the case the phenomenon of death
was inconspicuous, for that which died was very small. But as the
somatic cells relatively increased, the body became of more importance
as compared with the reproductive cells, until death seems to affect
the whole individual, as in the higher animals, from which our ideas
upon the subject are derived. In reality, however, only one part
succumbs to natural death, but it is a part which in size far surpasses
that which remains and is immortal,—the reproductive cells.
Götte combats the statement that the idea of death necessarily implies
the existence of a corpse. Hence he maintains that the cellular sac
which is left after the extrusion of the reproductive cells among the
Orthonectides, and which ultimately dies, is not a corpse; ‘for it does
not represent the whole organism, any more than the isolated ectoderm
of any other Heteroplastid’ (l. c., p. 48). But it is only a popular
notion that a corpse must represent the entire organism. In cases of
violent death this idea is correct, because then the reproductive cells
are also killed. But as soon as we recognise that the reproductive
cells on the one side, and the somatic cells on the other, form
respectively the immortal and mortal parts of the Metazoan organism,
then we must acknowledge that only the latter,—that is, the _soma_
without the reproductive cells,—suffers natural death. The fact that
all the reproductive cells have not left the body (as sometimes
happens) before natural death takes place, does not affect this
conception. Among insects, for instance, it may happen that natural
death occurs before all the reproductive cells have matured, and these
latter then die with the _soma_. But this does not make any difference
to their potential immortality, any more than it modifies the
scientific conception of a corpse. The idea of natural death involves
that of a corpse, which consists of the _soma_, and when the latter
happens to contain reproductive cells, these do not succumb to a
natural death, which can never apply to them, but to an accidental
death. They are killed by the death of the _soma_ just as they might be
killed by any other accidental cause of death.
The scientific conception of a corpse is not affected, whether the dead
_soma_ remains whole for some time, or falls to pieces at once. I
cannot therefore agree with Götte when he denies that an Orthonectid
possesses ‘the possibility of becoming a corpse’ (in his sense of the
word) because ‘its death consists in the dissolution of the structure
of the organism.’ When the young of the Rhabdites form of _Ascaris
nigrovenosa_ bore through the body-walls of their parent, cause it to
disintegrate and finally devour it, the whole organism disappears, and
it would be difficult to say whether a corpse exists in the popular
sense of the word. But, scientifically speaking, there is certainly a
corpse; the real _soma_ of the animal dies, and this, however
subdivided, must be considered as a corpse. The fact that natural death
is so difficult to define without any accurate conception of what is
meant by a corpse, proves the necessity for arriving at a scientific
idea as to the meaning of the latter. There is no death without a
corpse—whether the latter be small or large, whole or in pieces.
If we compare the bodies of the higher Metazoa with those of the lower,
we see at once that not only has the structure of the body increased in
size and complexity as far as the _soma_ is concerned, but we also see
that another factor has been introduced, which exercises a most
important influence in lengthening the duration of life. This is the
replacement of cells by multiplication. Somatic cells have acquired (at
any rate in most tissues) the power of multiplying, after the body is
completely developed from the reproductive cells. The cells which have
undergone histological differentiation can increase by fission, and
thus supply the place of those which are being continually destroyed in
the course of metabolism. The difference between the higher and lower
Metazoa in this respect lies in the fact that there is only one
generation of somatic cells in the latter, and these are used up in the
process of metabolism at almost the same time that the reproductive
cells are extruded, while among the former there are successive
generations of somatic cells. I have elsewhere endeavoured to render
the duration of life in the animal kingdom intelligible by the
application of this principle, and have attempted to show that its
varying duration is determined in different species by the varying
number of somatic cell-generations[86]. Of course, the varying duration
of each cell-generation materially influences the total length of life,
and experience teaches us that the duration of cell-generations varies,
not only in the lowest Metazoa as compared with the highest, but even
in the various kinds of cells in one and the same species of animal.
We must, for the present, leave unanswered the question—upon what
changes in the physical constitution of protoplasm does the variation
in the capacity for cell-duration depend; and what are the causes which
determine the greater or smaller number of cell-generations. I mention
this obvious difficulty because it is the custom to meet every attempt
to search deeper into the common phenomena of life with the reproach
that so much is still left unexplained. If we must wait for the
explanation of these processes until we have ascertained the molecular
structure of cells, together with the changes that occur in this
structure and the consequences of the changes, we shall probably never
understand either the one or the other. The complex processes of life
can only be followed by degrees, and we can only hope to solve the
great problem by attacking it from all sides.
Therefore it is, in my opinion, an advance if we may assume that length
of life is dependent upon the number of generations of somatic cells
which can succeed one another in the course of a single life; and,
furthermore, that this number, as well as the duration of each single
cell-generation, is predestined in the germ itself. This view seems to
me to derive support from the obvious fact that the duration of each
cell-generation, and also the number of generations, undergo
considerable increase as we pass from the lowest to the highest Metazoa.
In an earlier work[87] I have attempted to show how exactly the
duration of life is adapted to the conditions by which it is
surrounded; how it is lengthened or shortened during the formation of
species, according to the conditions of life in each of them; in short,
how it is throughout an adaptation to these conditions. A few points
however were not touched upon in the work referred to, and these
require discussion; their consideration will also throw some light upon
the origin of natural death and the forms of life affected by it.
I have above explained the limited duration of the life of somatic
cells in the lower Metazoa—Orthonectides—as a phenomenon of adaptation,
and have ascribed it to the operation of natural selection, at the same
time pointing out that the existence of immortal Metazoan organisms is
conceivable. If the Monoplastides are able to multiply by fission,
through all time, then their descendants, in which division of labour
has induced the antithesis of reproductive and somatic cells, might
have done the same. If the Homoplastid cells reproduced their kind
uninterruptedly, equal powers of duration must have been possible for
the two kinds of Heteroplastid cells; they too might have been immortal
so far as immortality only depends upon the capacity for unlimited
reproduction.
But the capacity for existence possessed by any species is not only
dependent upon the power within it; it is also influenced by the
conditions of the external world, and this renders necessary the
process which we call adaptation. Thus it is just as inconceivable that
either a homogeneous or a heterogeneous cell-colony possessing the
physiological value of a multicellular individual should continue to
grow to an unlimited extent by continued cell-division, as it is
inconceivable that a unicellular being should increase in size to an
unlimited extent. In the latter case the process of cell-division
imposes a limit upon the size attained by growth. In the former, the
requirements of nutrition, respiration, and movement must prescribe a
limit to the growth of the cell-colony which constitutes the individual
of the higher species, just as in the case of the unicellular
Monoplastides, and it does not affect the argument if we consider this
limitation to be governed by the process of natural selection. It would
only be possible to regulate the relations of the single cells of the
colony to each other by fixing the number of cells within narrow
limits. During the development of _Magosphaera_—one of the
Homoplastides—the cells arrange themselves in the form of a hollow
sphere, lying in a gelatinous envelope. But the fact that reproduction
does not follow the simple unvarying rhythm of unicellular organisms is
of more importance; for a rhythm of a higher order appears, in which
each cell of the colony separates from its neighbours, when it has
reached a certain size, and proceeds by very rapid successive divisions
to give rise to a certain number of parts which arrange themselves as a
new colony. The number of divisions is controlled by the number of
cells to which the colony is limited, and at first this number may have
been very small. With the introduction of this secondary higher rhythm
during reproduction, the first germ of the Polyplastides became
evident; for then each process of fission was not, as in unicellular
organisms, equivalent to all the others; for in a colony of ten cells
the first fission differs from the second, third, or tenth, both in the
size of the products of division and also in remoteness from the end of
the process. This secondary fission is what we know as segmentation.
It seems to me of little importance whether the first process of
segmentation takes place in the water or within a cyst, although it is
quite possible that the necessity for some protective structure
appeared at a very early period, in order to shield the segmenting cell
from danger.
It is impossible to accept Götte’s conception of the germ (Keim), and
at this point the question arises as to its true meaning. I should
propose to include under this term every cell, cytode, or group of
cells which, while not possessing the structure of the mature
individual of the species, possesses the power of developing into it
under certain circumstances. The emphasis is now laid upon the
expression development, which is something opposed to simple growth,
without change of form. A cell which becomes a complete individual by
growth alone is not a germ but an individual, although a very small
one. For example, the small encapsuled Heliozoon, which arises as the
product of multiple fission, is not a germ in our sense of the word. It
is an individual, provided with all the characteristic marks of its
species, and it has only to protrude the retracted processes
(pseudopodia) and to take in the expelled water (formation of vacuoles)
in order to become capable of living in a free state. In this sense of
the word, germs are not confined to the Polyplastides, but are found in
many Monoplastides. There is nevertheless, in my opinion, a profound
and significant difference between the germs of these two groups. And
this lies not so much in the morphological as in the developmental
significance of these structures. As far as I have been able to compare
the facts, I may state that the germs of the Monoplastides are entirely
of secondary origin, and have never formed the phyletic origin of the
species in which they are found. For instance, the spore-formation of
the Gregarines resulted from a gradually increasing process of
division, which was concentrated into the period of encystment; and it
was induced by a necessity for rapid multiplication due to the
parasitic life and unfavourable surroundings of these animals. If
Gregarines were free-living animals, they would not need this method of
reproduction. The encysted animal would probably divide into eight,
four, or two parts, or perhaps, like many Infusoria[88], it would not
divide at all, so that the whole reproduction would depend on simple
fission alone during the free state.
The original mode of reproduction among the Monoplastides was
undoubtedly simple fission. This became connected with encystment,
which originally took place without multiplication; and only when the
divisions in the cyst became excessively numerous did such minute
plastids appear that a genuine process of development had to be
undergone in order to produce complete individuals. Here we have the
general conception of the germ as I defined it. Its limitations are
naturally not very sharply defined, for it is impossible to draw an
absolute distinction between simple growth and true development
accompanied by changes in form and structure. For instance, Häckel’s
_Protomyxa aurantiaca_ divides within its cyst into numerous plastids,
which might be spoken of as germs. But the changes of form which they
undergo before they become young _Protomyxae_ are very small, and for
the most part depend upon the expansion of the body, which existed in
the capsule as a contracted pear-shaped mass. It is therefore more
correct to speak only of the simple growth of the products of the
fission of the parent organism, and to look upon these products as
young _Protomyxae_ rather than germs. On the other hand, the young
animals which creep out of the germs (the ‘spores’) of _Gregarina
gigantea_, described by E. van Beneden, differ essentially from the
adult, and pass through a series of developmental stages before they
assume the characteristic form of a Gregarine.
This is true development[89]. But such a method of germ-formation and
development are found most frequently, although not exclusively, among
the parasitic Monoplastides, and this fact alone serves to indicate
their secondary origin. It is a form of ontogenetic development
differing from that of the Polyplastides in that it does not revert to
a phyletically primitive condition of the species, but, on the
contrary, exhibits stages which first appear in the phyletic
development of the specific form. The Psorosperms were only formed
after the Gregarines had become established as a group. The amoeboid
organisms which creep out of them are in no way to be regarded as the
primitive forms of the Gregarines, even if the latter may have
resembled them, but they are coenogenetic forms produced by the
necessity for a production of numerous and very minute germs. The
necessity for a process of genuine development perhaps depends upon the
small amount of material contained in one of these germs, and on other
conditions, such as change of host, change of medium, etc. It therefore
results that the fundamental law of biogenesis does not apply to the
Monoplastides; for these forms are either entirely without a genuine
ontogeny and only possess the possibility of growth, or else they are
only endowed with a coenogenetic ontogeny[90].
Some authorities may be inclined to limit the above proposition, and to
maintain that we must admit the possibility that we are likely to
occasionally meet with an ontogeny of which the stages largely
correspond with the most important stages in the phyletic development
of the species, and that the ontogenetic repetition of the phylogeny,
although not the rule, may still occur as a rare exception in the
Protozoa.
A careful consideration of the subject indicates, however, that the
occurrence of such an exception is very improbable. Such an ontogeny
would, for instance, occur if one of the lowest Monoplastides, such as
a Moneron, were to develope into a higher form, such as one of the
Flagellata, with mouth, eye-spot, and cortical layer, under such
external conditions that it would be advantageous for the existence of
its species that it should no longer reproduce itself by simple
fission, but that the periodical formation of a cyst (which was perhaps
previously part of the life-history) should be associated with the
occurrence of numerous divisions within the cyst itself, and with the
formation of germs. We must suppose either that these germs were so
minute that the young animals could not become Flagellata directly, or
that it was advantageous for them to move and feed as Monera at an
early period, and to assume the more complex structure of the parent by
gradual stages. In other words, the phyletic development would proceed
hand in hand with the ontogeny corresponding to it, although not from
any internal cause, but as an adaptation to the existing conditions of
life. But the supposed transformation of the species also depended upon
these same conditions of life, which must therefore have been of such a
nature as to bring about simultaneously, by an intercalation of germs
and by a genuine development, the evolution of the form in question in
the last stage of its ontogeny, and the maintenance of its original
condition during the initial stage. Such a combination of circumstances
can have scarcely ever happened. Against the occurrence of such a
transformation as we have supposed, it might be argued, indeed, that
the assumed production of very numerous germs does not occur among
free-living Monoplastides. Those which have acquired parasitic habits
must be younger phyletic forms, for their first host—whether a lowly or
a highly organized Metazoon—must have appeared before they could gain
access to it and adapt themselves to the conditions of a parasitic
life, and by this time the Flagellate Infusoria were already
established. It is by far less probable that the persistence or rather
the intercalation of the ancestral form would occur in an ontogenetic
cycle, consisting of a series of stages, and not of two only, as in our
example. For as soon as reproduction can be effected by the simple
fission of the adult, not only is there no reason why the earlier
phyletic stages should be again and again repeated, but such
recapitulation is simply impossible. We cannot, therefore, conclude
that the anomalous early stages of a Monoplastid such as _Acineta_
correspond with an early form of phyletic development.
Supposing, for instance, that the Acinetaria were derived from the
Ciliata, then this transformation must have taken place in the course
of the continued division of the ciliate ancestor—partially connected
with encystment, but for the most part independently of it. Of the
myriads of generations which such a process of development may have
occupied, perhaps the first set moved with suctorial processes, while
the second gradually adopted sedentary habits, and throughout the whole
of the long series, each succeeding generation must have been almost
exactly like its predecessor, and must always have consisted of
individuals which possessed the characters of the species.