Part 37

It was by chance that human observation brought the latter fact to light first, and therefore led us for so long to accept the idea that _fertilization_, that is, amphimixis, and _development_, that is, reproduction, are one and the same; and thus it happens that even now there are many naturalists who cannot rid themselves of the idea that amphimixis, if not a life-awakening, is at least a _life-renewing_ process, a so-called 'process of rejuvenescence.'

More than ten years ago[14] I disputed this view, and since then the facts which make it untenable have become more and more clear. Notwithstanding this I see that it is still adhered to, at least in a modified form, by many esteemed naturalists, and so it does not seem superfluous to discuss it in more detail.

[14] _Die Bedeutung der sexuellen Fortpflanzung für die Selektionstheorie_, Jena, 1886.

I have already noted that we see in conjugation an amphimixis without reproduction, and in spores and parthenogenetic ova reproduction without amphimixis, and I do not doubt that every unprejudiced critic will admit this; many among us, however, are not unprejudiced, but are under the spell of earlier ideas, so that they cannot forget that it was long believed that fertilization was an indispensable condition of development; they therefore regard the divisions which recommence sooner or later after conjugation, and which may be repeated hundreds of times, _as conditioned by the conjugation which preceded them_, and compare them to the series of cells which, in the Metazoa, lead from the fertilized ovum to the fully-formed animal. They regard both series of cell-generations as a developmental cycle, which leads from fertilization to fertilization again, from conjugation to conjugation, and which would be impossible without either fertilization or conjugation.

This play with the idea of a 'cycle' reminds me vividly of similar fantastic play from the time of the much-despised 'Naturphilosophie' of a hundred years ago. As men sought to find the analogues of 'solar' and 'planetary' systems in animal and plant, and believed they had stated something when they compared the motile animals to planets and the sedentary plants to the sun (!), so it is now imagined that a deeper insight has been gained by the recognition of cycles of development. By all means let us regard the development of a multicellular organism as cyclic; it returns again to its starting-point, but this no more explains the forces which produce the cycle, and thus the meaning of fertilization, than a comparison with the circling planets explains the causes of locomotion in animals. With quite as much reason the cycle of development might be made to start from the parthenogenetic ovum, and then the whole conclusion of the fanciful cycle idea in regard to the meaning of fertilization falls to the ground, for in this case the cycle begins without fertilization. Attempts are made to get over this difficulty by showing that in many cases parthenogenesis alternates regularly or irregularly with sexual reproduction, as in the water-fleas (Daphnids), the Aphides, and so on. The mysterious rejuvenating power of amphimixis is supposed to suffice for several generations, a purely gratuitous assumption, which is also in open contradiction to the facts. For there are species which now reproduce exclusively by parthenogenesis, among plants for instance, a number of fungi, among animals a few species of Crustaceans. Of the latter it can be demonstrated that ages ago they reproduced sexually, for they still possess the sac which serves for receiving spermatozoa, but this sac remains empty, for there are now no males, at least in any habitat of the species known to us. To this set belongs an inhabitant of stagnant water, _Limnadia hermanni_, a species of Crustacean which was found thirty years ago in hundreds, all of the female sex, near Strassburg, and also many of the little Ostracods (_Cypris_) which inhabit especially the muddy bottom of our pools and marshes. I bred one of these (_Cypris reptans_) in numerous aquaria for sixteen years, during which there were about eighty generations, and throughout this time no male ever appeared, nor did the sperm-sac of the female ever contain spermatozoa. The after-effects of the 'rejuvenating' power of an amphimixis supposed to have taken place earlier must in this case have been enduring indeed!

For these reasons it seems to me useless to make comparisons between the developmental cycle of unicellular organisms and the ontogeny of multicellular organisms. Both processes have indeed many points of resemblance--long series of cells, then interruption of the divisions and the occurrence of amphimixis--so that we may quite well speak of cyclic development in the physiological sense, in as far as certain internal conditions periodically recur and compel the organism to conjugation, but we must not suppose that there is more in this than, for instance, in the 'cyclic development' of Man, which consists in the fact that he finds himself periodically impelled to take food. The feeling of hunger which forces him to do so is the signal which warns the organism that it is time to supply fresh combustible material to the metabolism. In the same way, after a long series of generations of Infusorians the necessity for conjugation arises; the whole colony suffers an 'epidemic of conjugation,' and the animals unite in pairs; in the meantime we know not why, and must content ourselves with formulating what is observable, that _the nuclear substances of two individuals are thereby mingled in each conjugate_.

Obviously the impulse to conjugation is a signal in the same sense as the feeling of hunger is, and we know well from the higher animals what a mighty influence it exerts, an influence hardly less potent than that of hunger. In Schiller's words, 'Durch Hunger und durch Liebe, erhält sich dies Weltgetriebe.'

We can see clearly enough why Nature should have given animals the feeling of hunger, but the reason for the need of conjugation is not so plain; we can only say in the meantime that it must be of some value in maintaining the forms of life, for only that which fulfils a purpose can be permanently established.

I shall return later to the problem of the meaning of 'sexual 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.