reproduction,' and try to probe more deeply into the meaning of its
establishment; in the meantime I must restrict myself to having shown
its significance in the union of the hereditary substances of two
individuals, and at the same time to controverting the theory of the
'rejuvenating power' of amphimixis. I use this expression in its
original sense, which indicates that every life is gradually wearing
itself away and would become extinct were it not fanned to flame again
by amphimixis--by an artifice of Nature, we may say. This conception
rests on the fact that the cells of the multicellular body possess for
the most part only a limited length of life, for they are used up by
the processes of life, and they break up and die, some sooner, some
later. As it is observed that all true somatic cells, among higher
animals at least, are subject to this law of mortality, but that the
germ-cells are not, and that, furthermore, the germ-cells only develop
when they are fertilized, the cause of the potential immortality of
the germ-cells is believed to lie in amphimixis, and a 'rejuvenating'
power in fertilization, or, more generally, in amphimixis, is inferred.
Mystical as this sounds, and little as it agrees with our otherwise
mechanical conceptions of the economy of life, it was until very
recently a widespread view, although perhaps it is now abandoned by
many who formerly held it, and has been imperceptibly modified into
a quite different conception, for which the word 'rejuvenescence' is
retained, but with the altered meaning of a mere 'strengthening of the
metabolism' or 'of the constitution.' By many authors, indeed, the two
meanings of the word are not clearly kept apart. I shall return later
to the modified meaning of the word 'rejuvenescence,' and shall keep
in the meantime to the original meaning of the word, which implies a
renewal of life which would otherwise die out.
This meaning seemed to gain a firm hold, when, about fifteen years ago,
the French investigator Maupas published his remarkable observations
on the conjugation of Infusorians. These seemed to show that colonies
of Infusorians which were artificially prevented from conjugating
gradually died out; not of course at once, but after many, often
several hundred, generations; ultimately a degeneration of all the
animals in such colonies set in, and ended only with their utter
extinction. Maupas himself interpreted this as _a senile degeneration_
which took place because conjugation had been prevented, and he
therefore regarded conjugation as a '_rajeunissement karyogamique_,'
a rejuvenescence, and therefore a means of preventing the ageing and
final dying off of the individuals--of obviating, in short, the natural
death to which in his opinion they would otherwise be subject. This
conception was greeted with general approval, and there are many people
who still regard conjugation as a process by which the capacity for
life is renewed--a view which I must still dispute as emphatically as I
did some years ago.
In the first place, the observations on which this theory is based
admit of another interpretation, quite different from that which has
been assumed to be the only possible one. Maupas prevented conjugation,
not perhaps because he had isolated individuals and their progeny, but
by exposing the whole colony of near relatives to unusual conditions
when conjugation was just about to set in, namely, by supplying
them with particularly abundant food. The need for conjugation then
disappeared, as, conversely, it could be called forth at any time in
a colony by hunger. But these are artificial conditions, and indeed
the breeding of Infusorians for months in a small quantity of water on
the object-glass certainly does not correspond to natural conditions.
We must admire the skill of the investigator who was able to keep his
colonies alive for months and years under such artificial conditions,
but we may venture to doubt whether the fate of extinction which did
ultimately overtake them was really due to the absence of conjugation,
and not to the unnaturalness of the conditions.
In any case a repetition and modification of Maupas' experiments is
very desirable, and would be of lasting value[15].
[15] Since the above was written Calkins has made a series of new
experiments, the results of which differed in several respects from
those yielded by Maupas' experiments. When his infusorian-cultures
began to grow weaker, as happened frequently and at irregular
intervals, he was always able to restore them to more vigorous life by
a change of diet, and especially by substituting grated meat, liver,
and the like for infusions of hay. Certain salts, too, had the same
effect: the animals became perfectly vigorous again. Calkins believes
that chemical agents, and especially salts, must be supplied to the
protoplasm from time to time. He reared 620 generations of Paramœcium
without conjugation. But the 620th was weakly and without energy. The
addition of an extract of sheep's brains made them perfectly fresh
and vigorous again. Further experiments in this direction are to be
desired, but, according to those of Calkins, it is probable that
Infusorians can continue to live for an unlimited time even without
conjugation.
Let us, however, assume for the moment not only that Maupas'
observations were correct, which I do not doubt, but also that they
were rightly interpreted. Would they in that case afford a proof that
amphimixis means a rejuvenescence of the power of life? To my thinking,
not in the remotest degree.
It certainly seems as if this were true at the first glance; the
colony which is prevented from conjugating goes on multiplying for a
considerable time, often indeed for hundreds of generations, but this
may be compared with sufferers from hunger, whose life does not cease
at once if the feeling of hunger is not appeased.
It was certainly made evident by these experiments that Infusorians
which were prevented from conjugating were incapable of unlimited
persistence. But even this in no way proves that amphimixis has a
power of rejuvenating life, but simply that these animals are adapted
for conjugation, and that they degenerate without it, just as the
sperm-cell or the ovum dies if it does not attain to amphimixis.
My opponents take it as axiomatic that the life-movement must come to
a standstill of itself, and that it therefore requires help. Even so
distinguished a specialist on the Protozoa as Bütschli argues that
organisms are not _perpetua mobilia_, and when one remembers the
physicist's theory of the impossibility of a _perpetuum mobile_ this
looks at first sight like a formidable objection. But does the organism
always remain the same as long as it lives, like a pendulum which
friction causes to swing more and more slowly till ultimately it comes
to a standstill? We know surely that the phenomena of life arise from
a continual process of combustion, which is followed by a constant
replacement of the used-up particles by new particles; we know that
life depends on an unceasing metabolism, which brings about changes
in the material basis of the organism every moment, so that it is
constantly becoming new again.
I shall attempt to show later on that the cells cannot be the ultimate
elements of the organism, but that the life-units visible with
the microscope must be made up of smaller invisible units. These,
therefore, undergo 'metabolism,' which conditions their multiplication
and their destruction, and this 'metabolism' is not to be seen only in
the building up and breaking down of 'albuminoid substances,' as the
physiologists say, but in the alternation between the multiplication
and the dissolution of these smallest vital particles. Therefore, it
seems to me that the movement of life, whether in a single-celled or
in a many-celled organism, is not to be compared to one pendulum,
but to an endless number of pendulums which succeed one another
imperceptibly in the course of the metabolism, always producing anew
the same result, which therefore may continue _ad infinitum_. Suppose,
then, that we possessed our present conception of life as a process of
combustion, and of metabolism as the agency which continually provides
new combustible material in the shape of new vital particles, but that
we knew nothing about multicellular organisms and their transitory
existence, but were acquainted only with unicellular organisms and
their unlimited multiplication by division. If we were then to make
the observation that all multicellular organisms are mortal, subject
to natural and inevitable death, it would at first appear to us quite
unintelligible, since we should be aware that in these also the fire
of life is continually being fed by the supply of new combustible
material. Not the potential immortality of unicellular organisms would
then appear to us remarkable and surprising, but the limitation of the
life of multicellular organisms--the occurrence of natural death. Who
knows whether, in that case, many of those investigators trained in
regard to unicellular organisms alone would not say just the opposite
of what Bütschli has said, that there could be no natural death in
many-celled organisms, since single-celled organisms prove to us that
life is an endless chain of transitory minute vital units?
Furthermore, our physiologists are still far from being able to explain
the natural death of many-celled organisms from below--I mean from a
knowledge of its necessary causes; on the contrary, they argue from
the known occurrence of natural death to the causes which underlie
it; and thus they have arrived at the idea, undoubtedly correct, that
the somatic cells of the body are gradually so altered by their own
activity that they are ultimately unable to function any longer and
must die off. Therefore, if we were unacquainted with death, we should
not have been able to infer it from our physiological knowledge, and
still less from our knowledge of the unicellulars.
As our insight has in point of fact grown by starting from the
mortal many-celled organisms, and has only later penetrated down to
the unicellular organisms, so we can understand the genesis of the
conclusion, deduced from the mortality of the many-celled organisms,
that unicellular organisms also are unable to continue without limit
the renewal of material and of vital particles, and that consequently
they would be subject to natural death if nature had not found in
conjugation a 'remedy' for 'the physiological difficulties which ensue
automatically and necessarily from the constitution and from the
continual functioning' even of unicellular organisms.
But we ask in vain for a shadow of proof of this remarkable conception;
it is an axiom deduced from our knowledge of natural death among
multicellular organisms, and bolstered up by a mistaken application of
the idea of 'perpetual motion.' Or may we regard it as a proof of this
if it should be found that all unicellular organisms are adapted for
conjugation?
We shall see later on that amphimixis has certainly quite a different
and, undoubtedly, a very important effect, namely, that it increases
the capacity of the species for adaptation; and a life-renewing
effect in Bütschli's sense could only be ascribed to it in addition
if the assumption of the necessity of a natural death in unicellular
organisms were not directly contrary to the clear facts of the case;
but this is just what it is.
We are acquainted with such contradictory facts, not perhaps among
the unicellulars themselves, where it is difficult to procure direct
proof, but in regard to the germ-cells of many-celled organisms which
correspond to unicellular organisms. We know that under certain
circumstances the ovum is capable of persisting by itself--in cases
of parthenogenesis--how then can we conclude that amphimixis is in
the case of Metazoan germ-cells the cause of their capacity for
development? We can only conclude, it seems to me, that their power of
developing is usually bound up with the occurrence of amphimixis. So we
may conclude in regard to the unicellulars that their unlimited power
of multiplication is bound up with the occurrence of amphimixis, but
not that amphimixis is the cause of this power, or that it implies a
rejuvenescence of life. If unicellular organisms could have been made
immortal through amphimixis, then what I maintain would be proved--that
they possess potential immortality; but if they did not possess it, no
artifice in the world could give it to them; amphimixis could be at
most only the condition with the fulfilment of which the realization of
their immortality was bound up.
One may ask, How then can amphimixis be a condition of their survival?
why should Infusorians which have not conjugated at the proper time be
doomed to extinction? And from the standpoint of our present knowledge
I am as little able to give a precise answer as my opponents. But I
can give one in relation to the amphimixis of multicellular organisms,
for in regard to these we know that each of the germ-cells--male and
female--uniting in fertilization, is of itself incapable of development
and doomed to perish, the sperm-cell because it is too small in mass
to be able to develop the whole organism, and the ovum because, in
order to become capable of being fertilized, it must undergo certain
changes which make it incapable of independent development. We have
seen that after the two maturing divisions in the egg-cell have been
accomplished the ovum no longer contains a mechanism of division,
as the centrosphere breaks up after the second division; embryonic
development can therefore only begin when a new centrosphere has
been introduced into the ovum, and this is normally brought about
by fertilization, that is, by the entrance of the sperm-cell, whose
nucleus is accompanied by a centrosphere.
Thus amphimixis is seen to be really a condition of development. But
we now know that the ovum can emancipate itself from this condition,
by only going through a part of the processes of maturation which are
related to the subsequent amphimixis, and by thus retaining its own
centrosome. Nothing is more instructive in this connexion than the
cases we have already briefly discussed of facultative or occasional
parthenogenesis. We have seen that in some insects, for instance in
the silk-moths, there are sometimes, among thousands of unfertilized
eggs, a few that develop little caterpillars. If we examine a large
number of such unfertilized eggs we not infrequently find among them
several which, although they have not gone through the whole course of
development, have at least gone through the earlier stages, and others
which may have advanced somewhat further and then come to a standstill;
in short, we can see that several of these eggs were capable of
parthenogenetic development, although in varying degrees.
The cause of this parthenogenetic capacity has not as yet been
definitely determined by observation, but we shall hardly go wrong
if we seek it in the fact that the centrosphere of the ovum does
not always perish immediately and completely during maturation, and
may persist, rarely in its integrity, but sometimes in a weakened
state. Future observations will probably reveal some differences in
the size or aster-forming power of the centrospheres of such eggs;
in any case it is of the greatest interest that stimuli of various
kinds--mechanical or chemical--can strengthen the disappearing
centrosphere of the ovum, although as yet we are far from being able to
say how this comes about.
The experiments already mentioned of Tichomiroff, Loeb, and Winkler
give us at least an indication how we must picture to ourselves the
origin of parthenogenesis, namely, through the fact that the breaking
up of the apparatus for division, introduced for the sake of compelling
amphimixis, is prevented. Minute changes in the chemistry of the ovum,
similar to those caused artificially in the ova of the sea-urchin by
the introduction of an infinitesimal quantity of chloride of magnesium
(Loeb), in the ovum of the silk-moth by friction or by sulphuric acid
(Tichomiroff), or in the sea-urchin ovum by an extract of the sperm of
the same animal (H. Winkler), will effect this modification, and normal
parthenogenesis is induced.
For the ovum, therefore, amphimixis is certainly not a life-renewing or
rejuvenating factor; it only appears as such because the process has
in the course of nature been made compulsory by making the two uniting
cells each incapable of developing by itself. As we have seen, this is
true also of the sperm-cell, for although it contains a centrosphere,
and would be capable of division as far as that is concerned, yet in
almost all animals and plants it consists of such a minimal quantity
of living matter that it is unable to build up a new multicellular
organism by itself. Only in one alga (_Ectocarpus siliculosus_) has
it been observed that not only the female germ-cell can develop
parthenogenetically under certain circumstances, but that the male-cell
may also do so. In this case, however, the difference in size between
the two is not great, and it is noteworthy that the male plant, in
correspondence with the smaller size of the zoosperm, tends to be a
somewhat poorly developed organism.
If we are forced to the conclusion in regard to multicellular organisms
that amphimixis does not supply the power of development to the ovum,
but that, on the contrary, the power of development is withdrawn
from the ovum, so that amphimixis can, so to speak, be forced, must
we not assume something similar for unicellular organisms also? May
not amphimixis be made compulsory in their case also, in that the
Infusorians in preparation for conjugation go through changes which
make their unlimited persistence possible only on condition that
they conjugate? In my opinion the division of labour in the nucleus,
which is differentiated into a macronucleus and a micronucleus, and
the transitory nature of the former, may be regarded as an adaptation
in this direction. In any case, it is striking that an organ which
otherwise persists without limit among unicellular organisms, the
nucleus, is here subject to natural death after the manner of the body
of multicellular organisms, that it breaks up and must be reformed
from the micronucleus which in this case is alone endowed with
potential immortality. I am inclined to regard this as an arrangement
for compelling conjugation, since it is only after conjugation that
the micronucleus forms a new macronucleus, although the latter
is indispensable to life, as we see from experiments in dividing
Infusorians artificially.
Suppose we had to create the world of life, and it was said to us that
amphimixis must--wherever possible--be secured periodically to all
unicellular and multicellular organisms, what better could we do than
arrange devices which should exclude individuals which, by chance or
constitution, could not attain to amphimixis from the possibility of
further life? But would amphimixis then be the cause of persistence or
a principle of rejuvenescence?
I do not see that there can be any ground for such an assumption
other than the tenacious and probably usually unconscious adherence
to the inherited and deep-rooted idea of the dynamic significance
of 'fertilization,' no longer, perhaps in its original form, which
regarded the sperm as the vital spark which awakened new life in the
dead ovum, but in the modified form of the 'rejuvenating' power of
amphimixis.
Quite recently an attempt has been made to modify the idea of the
'rejuvenating' effect of amphimixis so that it should mean only
an advantage, not an actual condition of persistence. Hartog, in
particular, admits so much, that the occurrence of purely asexual and
purely parthenogenetic reproduction excludes the possibility of our
regarding the process of amphimixis as a condition of the maintenance
of life. But then we must also cease to regard the 'ageing' and dying
off of Infusorians which have been prevented from conjugating as an
outcome of the primary constitution of the living substance, and should
entirely abandon the misleading expression 'rejuvenescence.'
If we fix our attention on the numberless kinds of cells in higher
organisms and on multicellular organisms as intact unities, we see that
they all die off, that they are subject to a natural death, that is, a
cessation of vital movement from internal causes, yet no one is likely
to refer their transitoriness to the fact that they do not enter into
amphimixis. We find it quite 'intelligible' that the cells of our body
should be used up sooner or later as a result of their own function,
though we are very far from being able to demonstrate the necessity for
this, and so really to 'understand' it.
It is only from the standpoint of utility that we can understand the
occurrence of natural death; we see that the germ-cells _must_ be
potentially immortal like the unicellular organisms, but that the
cells which make up the tissues of the body _may_ be transient, and
indeed _must_ be so in the interests of their differentiation--often
great and in one direction--which determines the services they render
to the body. They required to become so differentiated that they
could not continue to live on without limit, and they did become so
differentiated because only thus could an ever-increasing functional
capacity of the whole organism be rendered possible; but they die not
because 'rejuvenescence through amphimixis is denied them, but because
their physical constitution is what it is.' And we must explain the
death of the whole many-celled individual in a similar way. When we
were trying in a previous study to establish the unlimited continuance,
the potential immortality, of unicellular organisms, we noted that an
eternal continuance of the life of the body of multicellular organisms
could certainly not be a necessity, since the continuance of these
forms of life is secured by their germ-cells. A continuance of the
body cannot even be regarded as useful from any point of view. And
what is not useful for a form of life _does not arise as a lasting
adaptation_, which is of course not to say that an immortality of
multicellular organisms, such as they are now, would even have been
possible. If these organisms were to attain to such a high degree of
functional capacity and of structural complexity as they now exhibit,
they obviously could not also have been adapted at the same time to an
eternal persistence of life.
This is in perfect harmony with our whole conception of the impelling
forces in the development of the organic world; the ever-increasing
functional capacity of the structure arose from the advantage which
this afforded in the struggle for existence, in comparison with which
the apparent advantage of the endless life of the individual was of no
account whatever.
I will not here follow out this idea. I have merely touched on it in
order to make clear that the death of individuals in all multicellular
organisms gives us no ground for thinking of the unlimited life of
the germ-cells as dependent on a special artifice of nature, such as
amphimixis is often supposed to be. Let us always remember that there
is parthenogenesis, and that there are unicellular germs (spores)
which are never fertilized, and that the reproduction of many species
of animals and plants occurs in this way without the intervention of
amphimixis at all.
* * * * *