Fusion of this kind was demonstrated in the Myxomycetes almost forty years ago by De Bary, and it has been observed more recently in various unicellular organisms, especially in Rhizopods and in Heliozoa. These last often place themselves close together in pairs, threes, or even more at a time, and then the delicate cell-bodies coalesce, though no fusion of the nuclei takes place. With Hartog, we call this process 'Plastogamy,' but we cannot agree with that observer when he regards the importance of the process as consisting in the fact that the nuclei thus come into contact with fresh cell-substance, after having been surrounded for a very long time with the same cytoplasm. If this were the import of amphimixis, then an exchange of nuclei would take place, and this we find nowhere even among the lowest forms of life, for everywhere there is a union of the nuclear substance of two individuals. But this is by the way! Further cases of plastogamy have been observed in many of the limy-shelled Rhizopods. A union of this kind does not usually lead to any visible consequences, but in some Foraminifera a group of young animals is developed within the cell-bodies by the division of the nuclei and the cell-body; thus multiplication follows the fusion just as in perfect amphimixis, and we may therefore assume that there is a causal connexion between the two. In the slime-fungi, too, the union of several amœba-like cells into a multi-nucleated plasmodium is followed later by the development of numerous encapsuled spores, but only after the plasmodium, which to begin with is microscopically small, has grown to a macroscopically visible, reticulated mass (Æthalium) sometimes a foot in extent. In this case the fungus, creeping slowly over its foundation of decaying substance, takes up nourishment from it, and it is not possible to tell whether the union of the amœbæ yields any further advantage than that of facilitating the spreading over large uneven surfaces, and through this, later, the development of large fruit-bodies. But in the case of the Foraminifera the plastogamy has obviously another effect, unknown and mysterious, which as yet no one has ever been able to define precisely. Words like 'stimulus to growth,' 'stimulating of the metabolism,' and even 'rejuvenation,' give no insight into what happens, but that something happens, that through the fusion of two or more unicellulars a stimulus is exerted, which reveals itself later in increased rapidity of growth, we may, and indeed must assume, because this process has become a permanent arrangement in so many unicellular organisms. Only what is useful survives, and the uniting individuals must derive some advantage from the process of fusion, and it remains to be seen whether we can find out with any clearness what this advantage may be.
Till within a few decades ago it was believed that in this process one individual devoured the other, but this can now no longer be maintained. If any one still seriously considers this possible, Schaudinn's observations would convince him of his error, for in Trichosphærium, a marine, many-nucleated Rhizopod, he observed, on the one hand, the union of two or more animals, i. e. plastogamy, and on the other hand, the swallowing and digesting of a smaller member of the species by a larger one—two processes which are absolutely different, for in the first case the cell-bodies of the two animals remain intact, while an animal that is eaten becomes surrounded by a food-vacuole, and is dissolved and digested within it. In the former case the vital units (biophors) of each animal obviously remain intact and capable of function; in the second, those of the over-mastered animal are at once dissolved and chemically broken up; as biophors, therefore, they cease to exist. Whether one or the other process takes place may perhaps depend on whether the two animals differ greatly in size, so that the smaller can be quite surrounded by the larger.
In a former lecture I have emphatically expressed my dissent from the view which interprets amphimixis as a process of rejuvenation, meaning thereby a necessary renewal of life, and I need not go into this again in detail: for that the metabolism can continue through uncounted generations without being artificially stimulated—that is, in any other way than by nutrition—is proved by all those lowly organisms which exhibit neither plastogamy nor complete amphimixis, and also by the occurrence of purely parthenogenetic reproduction, &c. In what, then, can the advantage lie which the conjugating unicellular organisms derive from conjugation? Obviously not in that they impart to each other what each already possessed, but only in the communication of something special and individual, something that was peculiar to each, and becomes common to both.
Haberlandt believed that the development of auxo-spores in Diatoms pointed towards the processes which form the deepest roots of amphimixis. As is well known, the hard and unyielding flinty shell of these lowly Algæ involves a diminution of the organism at every division, so that the Diatoms become smaller and smaller as they go on multiplying, and if that went on without limit they would come rapidly to extinction. But a corrective is supplied in the periodical occurrence of conjugation of two organisms which have already materially diminished in size, and this is followed by the growth of the two fused individuals to the original normal size of the species.
It is, of course, obvious that in this case the union of two organisms which have become too small may be of advantage in bringing them back to the requisite normal size; but this is an isolated special case, which certainly does not justify our regarding conjugation as a means whereby diminished bodily size may be brought back to its normal proportions. By far the greater number of unicellular organisms are not permanently diminished in size by division, and even in the Diatoms the mass of the two fused individuals does not amount to the normal size of the species, so that even in this case there must be growth subsequent to the conjugation before the normal is re-attained. It may be doubted, therefore, whether the increase in mass is, even in the case cited, the essential event in conjugation, and whether there are not other effects which we cannot clearly recognize. Here, too, there must be differences between the two conjugating individuals, as we have just seen, for if they only communicated something similar to each other, the result would be an increase only in their mass, not in their qualities.
Although we cannot demonstrate differences of this kind in the case of the lowly organisms with which we are now dealing, we may assume their existence from analogy with the higher organisms. We know, especially through G. Jäger, that in Man every individual has a specific exhalation, his particular odour, and that in the secretions of his glands there are incalculably minute differences in chemical composition, which justify the conclusion that the living substance of the secreting cells themselves exhibits such differences, and that all the various kinds of cells in an individual are not absolutely identical with the corresponding cells of another individual, but that they are distinguished from them by minute yet constant chemical differences. The assumption that differences of this kind exist even in unicellulars, and in all lowly organisms generally, is not a merely fanciful one, but has much probability.
How far the combination of these individual differences of chemical, and at the same time vital, organization is able to quicken, to strengthen the metabolism, to bring about 'physiological regeneration,' or whatever we may choose to call it, we do not yet understand. It has been said that in plastogamy an exchange of 'substances' takes place; that each gives to the other the substances which it possesses and the other lacks, and that this causes an increase of vital energy. But it is unlikely that we have here to do merely with chemical substances, although these, of course, as the material basis of all vital processes, are indispensable; it seems to me more probable that the vital units (biophors) themselves in their specific individuality must play the chief part. But even this is saying very little, for we have not yet reached an understanding of these processes, and if we were not forced by the fact of plastogamy to the conclusion that this union must have some use, no one would have been likely to postulate it as useful, still less as necessary. It has, of course, been frequently suggested that multiplication by fission, if long-continued, results in 'exhaustion,' and that this is corrected by amphimixis, but who can tell why this 'exhaustion' might not be remedied, and even more effectually remedied, by a fresh supply of fuel, that is, of food? One might have thought that the vital processes would be thus more readily recuperated than by the co-operative combination of two already 'exhausted' cells. Two exhausted horses may perhaps be able to pull the load that one of them was no longer equal to, but in the case we are considering it is the combined burdens of two units that have to be borne, although each was no longer equal to its own share! That is more than we can understand.
Zehnder has recently defined the effect of amphimixis as a 'strengthening of the power of adaptation,' and he infers that the 'digestive fistellæ' (Biophors) of two individuals, which have somewhat different powers of digestion, are, when they combine, able to assimilate more kinds of food than either was able to assimilate by itself. But I confess that I do not see how an advantage for the whole would be gained through this alone, since half of the digestive biophors would have to work for the nutrition of the mass of the individual A, the other half of the differently constituted biophors for that of the individual B, and the nutritive capacity would thus remain exactly what it was before conjugation. Nevertheless I believe that Zehnder was right in his supposition that conjugation is concerned with strengthening the power of adaptation, and I have long maintained and defended this interpretation with regard to true amphimixis in nucleated organisms. In these cases it is quite obvious that the communication of fresh ids to the germ-plasm implies an augmentation of the variational tendencies, and thus an increase of the power of adaptation. Under certain circumstances this may be of direct advantage to the individual which results from the amphimixis, but in most cases the advantage will be only an indirect one, which may not necessarily be apparent in the lifetime of this one individual, but may become so only in the course of generations and with the aid of selection. For amphimixis must bring together favourable as well as unfavourable variations, and the advantage it has for the species lies simply in the fact that the latter are weeded out in the struggle for existence, and that by repetition of the process the unfavourable variational tendencies are gradually eliminated more and more completely from the germ-plasm of the species.
But this cannot have been the efficient cause in the introduction of amphimixis into the series of vital phenomena; the reason for this must be found in some direct advantage, such as that it improved and increased the assimilating power, the growth, and the multiplication of the particular individual, so that it gained an advantage over individuals which had not entered into conjugation. This advantage must exist, at least in the lower forms of conjugation, in pure plastogamy, i. e. in the mere coalescence of the protoplasmic bodies. But, as it seems to me, we have not yet clearly recognized what the advantage precisely is; we do not yet see how such a mingling or combination of two plasms should every time be of advantage for the combined conjugate. If we assume with Zehnder that two kinds of 'nutritive' biophors are brought together which differ slightly from each other in digestive capacity, three cases may occur. Either the food a, adequate for the animal A, is just as abundant as the food b, suitable for the animal B, and then half the conjugated animal will be nourished by means of the biophors a, the other half by means of the biophors b, and the state of matters is the same as it was before conjugation; or the food b is more abundant than the food a, or conversely, and then the biophors b will have to take the larger share in the nourishment of the conjugate A + B, and they will therefore multiply more rapidly and the biophors a will decrease relatively in number. Nutrition and growth will then go on more slowly for a time, but will soon attain to their former intensity. The combined individual A + B has then certainly gained an advantage over the isolated animal A, and the living substance of A which, if left to itself would probably have perished, can continue to live in combination with B. But in that case it is not obvious where the advantage in the union can lie, as far as B is concerned. An advantage to B only results if there be a combination not of one kind of biophor only, but of several or many kinds of biophors. If for instance A, whose digestive biophors were weak, brought with it into the partnership 'secretory' or nervous biophors stronger than those of B, then there would be an advantage for both in the combination, and it is thus that, in the meantime, I interpret the direct benefit which results from pure plastogamy. This benefit must be the more important and far-reaching the longer multiplication by fission continues without the occurrence of conjugation.
We thus reach what is perhaps a not wholly unsatisfactory conception of amphimixis, in so far at least that we do not require to assume that there has been a fundamental change in its significance between its expression in the lowest organisms and in the higher and even highest forms. Everywhere it is the same advantage: an increase in the power of adaptation; but it sometimes finds expression directly in the product of conjugation, sometimes only indirectly, sooner or later, among the descendants of the product.