LECTURE XXIX
THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (continued)
Association of amphimixis with reproduction—Origin of amphimixis—Its lowest forms—Amphimixis in Coccidia—Chromosomes in unicellular organisms—Coccidium proprium—'Amœba-nests' as a preliminary stage to amphimixis—Plastogamy of the Myxomycetes—Result: a strengthening of the power of adaptation—Strengthening of the power of assimilation—Use of complete amphimixis—Proof of its constant efficacy to be found in the rudimentary organs of Man—Allogamy—Means taken to prevent the mingling of nearly related forms—Amphimixis is not a 'formative' stimulus—Attraction of the germ-cells—Effects of inbreeding compared with those of parthenogenesis—Nathusius's case of injurious inbreeding—Hindrances to fertilization in the crossing of species—Probable reason for the injurious effect of inbreeding.
We have endeavoured to understand why amphimixis should have been established among the processes of life, and we have now to turn to the question when and how, that is, in what form, it was first introduced. But first I should like to refer for a little to the association of amphimixis with reproduction, which we find in all multicellular organisms, and among the higher types so unexceptionally that, until not very long ago, amphimixis and reproduction were looked on as one and the same thing, and all multiplication was believed to be associated with 'fertilization.' We have seen that this is not the case, that on the other hand the two processes are quite distinct, and may be called contrasts rather than equivalents, for reproduction always means an increase in the number of individuals, while amphimixis implies—originally at least—their diminution by a half.
Accordingly we found that, in unicellular organisms, amphimixis is not associated with reproduction, but interpolated between the divisions, and not even in such a manner that amphimixis precedes every multiplication by division, but so that the conjugation of two animals occurs only from time to time, after numerous divisions, sometimes hundreds, have occurred. It is obvious that this must be so, since, if amphimixis occurred regularly between every two divisions, no increase in the number of individuals would be brought about, at least if the fusion of the two conjugating individuals were complete.
Why, then, is there such an intimate, and in the case of the higher types, such an indissoluble, association between reproduction and amphimixis that 'fertilization' appears to be a sine qua non of reproduction, and not very long ago seemed to us to be the 'quickening of the ovum,' the 'burning spark' which causes the powder-barrel to explode?
The reason of this is not difficult to discover; it lies in the structure of multicellular animals, and in their differentiation according to the principle of division of labour, for since only particular cells are capable of reproduction, that is, of giving rise to the whole, it is in these necessarily that the process of amphimixis has to occur if its significance lies in its effects on the succeeding generations. It is true that in the lowest multicellular organisms, such as the species of Volvox, there are, in addition to the sex-cells, other reproductive cells quite similar to the ova, whose development into a new colony takes place without amphimixis, but the higher we ascend in the animal and plant series the rarer are these 'asexual' germ-cells or 'spores,' and in the highest animal types they are entirely absent and reproduction occurs only by means of the 'sex-cells.'
I am inclined to look for the cause of this striking phenomenon mainly in the fact that, if amphimixis had to be retained, this was effected with increasingly great difficulty the more highly and complexly differentiated the organisms became, and that more complicated adaptations were therefore necessary in order that the union of the two germ-cells might be rendered possible at all. There is first of all the separation into two kinds of sex-cells, whose far-reaching differentiations and precise adaptations to the most minute conditions we have already discussed; then follow the innumerable adaptations to bring about the meeting of the sex-cells, the arrangements for copulation, and, finally, the instincts which draw the two sexes together, the means of attraction which are employed, whether decorative colours or attractive shapes, stimulating odours or musical notes, in short, all the diverse and intricate arrangements, which seem to be more subtly elaborated the higher the organism stands upon the ladder of life. When we call to mind that sexual differentiations finally go so far that they dominate the whole organism, alike in its external appearance and in its internal nature, its feelings, inclinations, instincts, its will and ability, as well as its structure down to the finest nerve-elements, we can understand that a mode of reproduction which demands such a composite disposition of details, involving a moulding of the whole organism, so to speak, from birth till death, must of necessity remain the only one, and that there was no room for the persistence of any essentially different mode of reproduction with quite different adaptations. Or, to speak metaphorically, the power of adaptation which is innate in the organism so exhausted itself in the establishment of this marvellous amphimixis adjustment that the possibility of any other was totally excluded.
It is true that it is only among the Vertebrates that we find 'the reproductive apparatus' so highly developed, but even among Molluscs and Arthropods 'sexual' reproduction, that is, reproduction associated with amphimixis, is the prevailing mode. In these, indeed, parthenogenesis does occasionally occur, that is to say, sexually differentiated female germ-cells are, by means of some slight variations in the maturation of the egg, rendered capable of developing without previous amphimixis, but this happens only in quite special cases as an adaptation to quite special circumstances, and can only be regarded as a temporary cessation of the association between reproduction and amphimixis. In some cases it is a moiety of the ova adapted for amphimixis which develop parthenogenetically, as it is the same sexually differentiated animals, true females, which produce both sorts, and this is often true to some extent when the differentiation in the direction of parthenogenesis has advanced further, and the ova have been separated into those requiring fertilization and those which are parthenogenetic (e.g. the winter and the summer eggs of the Daphnidæ). Parthenogenesis is not asexual but unisexual reproduction, a mode of multiplication which shows us that even in highly differentiated animals the apparently indissoluble association between reproduction and amphimixis can be dissolved if circumstances require it.
But if amphimixis had to be retained in the higher animal forms—and we have seen reasons why this must be—it could only be effected by means of unicellular germs, for amphimixis is in essence a fusion of nuclei, and this is the reason why 'vegetative' reproduction, so-called, becomes less and less prominent in animals at least, and above the level of the Arthropods disappears almost entirely.
Let us now return to the question we asked at the beginning—When and in what form was amphimixis first introduced into the world of organisms? The best way to answer this is by observation. We must turn to the lowest forms which now exhibit it, and see whether it occurs in them in a simpler form, so that we may draw conclusions as to its origin and its primitive significance, for it would be possible, a priori, that this was something different from what it is now in the relatively higher organisms, and that a change of function has gradually come about.
Assuredly the whole intricate complex of adaptations which is now exhibited on the conjugation of the two sex-cells in animals and plants, the differentiation of two kinds of 'sexually' antagonistic cells, with all their special adaptations, the reduction of the chromosomes, the institution of the karyo-kinetic apparatus, together with the centrospheres and so on, cannot possibly have arisen all at once by fortuitous variation, but can only have arisen gradually, step by step, and as the result of 'innumerable external and internal influences.' But why should not these arrangements, nowadays so complex, have had a simple beginning? Why might not this beginning have been the simple union of the protoplasmic bodies of two non-nucleated Monera; followed, after the origin of nuclear substances, by the union of these, and, finally, after the differentiation of a nucleus with a definite number of chromosomes, with a dividing apparatus, with a membrane, and so on, by complete amphimixis as we now know it? And how many transition stages may not be added to fill up the gaps between these three main stages?
But how much we can actually prove in regard to these conceivable preliminary stages of amphimixis is another matter. If we take a survey of the observations that have been made up till now, we are confronted at first by the undoubtedly striking fact that very little is known about it as yet, for in fact the whole process is gone through even in quite lowly forms of life in a manner very similar to that in the higher forms. Amphimixis has been shown to be widespread even among unicellular organisms, yet not in an essentially simpler mode than among multicellulars. We have seen that even in ciliated Infusorians reducing division obtains, and that of the four nuclei which arise from twofold division of the original nucleus three break up again, and only the fourth, by a further division, separates into a male and a female pronucleus, 'which then complete the amphimixis with the corresponding pro-nuclei of another animal' (compare [Fig. 85], 4-7, vol. i. p. 321). This, and the existence of a dividing apparatus and of chromosomes, make the process appear very little less complicated than the fertilization of higher animals. The case is similar even in much lower unicellular organisms, such as Noctiluca ([Fig. 83], vol. i. p. 317). In this form and in Rhizopods it is true that reducing divisions have not yet been made out, but their occurrence in the lower Algæ (Basidiobolus), and above all in those simple unicellular organisms which give rise to malaria, and their allies, which live as 'Coccidia' in the blood-cells and intestinal cells of animals, leads us to expect that they may prove to be of general occurrence among unicellulars.
In the Coccidia, which are extremely simple unicellular organisms, equipped, however, with a nucleus, the adaptations relating to amphimixis are more extensive and more complex than in the Rhizopods. For while in the latter the two conjugating cells are absolutely alike in external appearance, in the former the male cell is distinct from the female, and indeed the differences are as marked as those that usually occur in multicellular animals.
Fig. 121. Life-cycle of Coccidium lithobii, a cell-parasite of the centipede Lithobius; after Schaudinn. 1, a 'sporozoite'; 2, the same penetrating into an intestinal epithelial cell; 3, the same growing into a 'schizont' capable of division; 4, the same dividing, and 5, breaking up into numerous pieces which separate from the 'residual body' in the centre, and either, as in 1, migrate into epithelial cells and repeat the history, or pass on to the phase of sexual reproduction. In the latter case, after eliminating a portion of the nucleus (reduction) in 6 and 6 a, they form the 'macrogamete' (the ovum); or within the mother-cell they produce microgametes (or sperm-cells), 7 and 7 a. The penetration of a sperm-cell into an egg-cell (amphimixis) is shown in 8, the fertilized egg-cell (9) becomes the so-called oocyst or permanent spore, from which by repeated division (10 and 11), new sporozoites, as in 1, arise, and begin the cycle afresh.
We owe our present knowledge of these processes especially to Schuberg, Schaudinn, and Siedlecki, and, because of their theoretical importance, I should like to summarize the essential points.
One of these Coccidia lives in the intestinal cells of a small centipede, Lithobius; in Fig. 121 the parasite is shown as a so-called 'Sporozoite,' that is, as a minute sickle-shaped cell, which at first moves freely about the intestine of the host (1), but then soon penetrates into an epithelial cell (2). There it grows to a spherical shape (3), and then, after having devoured the cell, it gives rise, by a peculiar process of division (Schizogony), to a number of very minute nucleated pieces, again sickle-shaped, the Schizonts, each of which bores its way into an epithelial cell as in 2, and follows the same path of development, so that a large number of cells in the intestine of the same host are attacked in this manner. But there is still another mode of reproduction, with which amphimixis is associated, which leads directly to the formation of 'lasting' germs which are enclosed in a capsule or cyst, reach the exterior with the excrement of the host, and thus spread the infection to other centipedes. The Schizonts which take this course develop into so-called macro-gametes and microgametes, the former being the female, the latter the male germ-cells. Then follows the penetration of a male gamete, actively motile because of its two flagella, into the female gamete (8). Amphimixis is accomplished, and the product of the fusion of the two sex-cells (9) surrounds itself with a thinner cyst, within which it multiplies by twofold division into four cells (10). These are the 'lasting' spores, which may dry up within the voided excrement of the centipede (11), and if they be eaten by another animal of the species, they infect it, for the sporozoites which have been formed by the previous divisions creep out, and in form 1 begin the life-history anew.
We have thus an alternation of four generations which are all unicellular, and of which one series (1-5) shows multiplication by fission, while the other (6-11) includes, besides multiplication by fission and as a condition of this, the process of amphimixis. Amphimixis must occur in order that the formation of 'lasting' spores and new sporozoites may result. We have thus a regular alternation of 'asexual' and 'sexual' reproduction, and the latter shows great resemblance to that of multicellular organisms. The macrogamete corresponds to the ovum, the microgametes to the spermatozoa, and they resemble these also in their greater numbers and in their structure.
But the resemblance goes even further. The ovum is much larger than the sperm-cell, and undergoes a kind of reduction of its nuclear substance; shortly before fertilization the ovum-nucleus ('the germinal vesicle') comes to the surface—just as in the case of animal ova—bursts, and extrudes a part of its substance in the form of a sphere (Fig. 121, 6 and 7). A reduction of the nuclear substance in the male cell has not been demonstrated in all cases, but in one of the Lithobius-Coccidia, Adelea ovata, the relatively large microgamete (the sperm-cell, Fig. 122, Mi) places itself close to one pole of the female macrogamete (the egg-cell) and then divides twice in succession, so that four small cells arise (Fig. 122, A-C); of these only one penetrates into the egg-cell (D, ♂K) and unites with it, the other three come to nought (D, Mi). What a surprising resemblance this bears to the twofold division of the mother sperm-cell in multicellular animals, through which the number of chromosomes is reduced to half! In the conjugation itself the thread-like chromosomes of the female nucleus are plainly recognizable, while those of the male remain coiled up (Fig. 122, D).
That the nuclear substance can be separated into chromosomes (ids) even in lowly unicellular organisms was probably first demonstrated by R. Hertwig for Actinosphærium, a Heliozoon or freshwater sun-animalcule, then by Lauterborn in regard to Diatoms, by Blochmann for an indigenous Rhizopod, Euglypha, and by Ishikawa for the marine Noctiluca. Fresh cases have been added in the last decade, so that we can now say that a considerable number of unicellulars, from the ciliated Infusorians and lower Algæ down to the Coccidia and Diatoms, exhibit a germ-plasm composed of ids. These structures behave in the same way as those in higher organisms, and Berger was able to demonstrate, in 1900, in the case of a Radiolarian, their multiplication by spontaneous splitting.
Fig. 122. Conjugation of a Coccidium (Adelea ovata), after Schaudinn and Siedlecki. A, the microgamete (sperm-cell) (Mi) has become closely apposed to the macrogamete (Ma). B, the reduction division of the nucleus of the macrogamete has been effected; Rk, directive corpuscles. In the microgamete the first division of the nucleus has begun. C, four nuclei in the microgamete, of which three come to nought. D, the fourth microgamete-nucleus (♂K) has become apposed to the nucleus of the ovum, in which distinct chromosomes are seen.
From our point of view all this cannot surprise us, since all these organisms, though only single cells, possess great complexity of structure; we need only call to mind the extremely fine differentiation of structure in numerous ciliated Infusorians, such as Stentor, which has already been mentioned, or the bell-animalcule (Vorticella) with its long and peculiarly ciliated gullet, its retractile ciliated disk, its muscular or myophane layer, its spirally retractile stalk with the ribbon-like, rapidly acting muscular axis; or the regular geometrically constructed flinty skeleton of the Radiolarians, with their radially disposed sword-like or rod-like needles and their complex interlacing lattice-work shells. In the latter case the complexity of the living substance becomes visible only through its product, the shell, for the protoplasm itself does not show any visible intricacy, and the same is true of the Coccidium whose life-history we have just been tracing, for in each of its stages it seems to be of very simple organization, though the succession of numerous different forms shows that its germ-substance must be composed of numerous determinants.
We cannot doubt, however, that, in all unicellular organisms, the protoplasm can be hardly less complicated as regards its minute invisible structure, since otherwise it would be impossible that the delicate vital processes which we observe in them should run their course. In this I agree, at least in principle, with the beautiful picture drawn by Ludwig Zehnder in his recent book[22] already mentioned, though he reached it in quite a different way, namely, by a purely synthetic method. He made the daring attempt to build up the organic world from below, starting from atoms and molecules, and ascending from these to the lowest vital units, our biophors, to which he attributes a tubular shape and therefore calls fistellæ. He imagines the cell to be made up of a large number, perhaps millions, of different kinds of fistellæ, of which one presides over the power of turgidity, another over endosmosis, a third over contraction, a fourth over the conduction of stimuli, &c., so that there results a high degree of cellular complexity, a composition out of numerous kinds of biophors arranged on a definite architectural plan. All this corresponds perfectly with the views I have so long championed, and which alone make the existence of a nucleus intelligible, if it is composed—as I assume—essentially of an accumulation of determinants, that is, of hereditary substances. And that such a high degree of complexity of structure is not a mere fanciful picture we see occasionally even in the case of unicellular organisms. Thus, for instance, in Coccidium proprium, parasitic in the newt (Triton), the macrogamete or egg-cell (Fig. 123, Ma) before fertilization by the sperm-cell or microgamete (Fig. 123, Mi) surrounds itself with a capsule, at one pole of which a minute opening, the micropyle, remains for the entrance of the male cell. This proves, it seems to me, that this particular spot of the capsule is hereditarily determined, just as much and just as definitely as the ray of the flint-skeleton of a Radiolarian. But if any spot of the capsule can vary by itself alone, may not numerous other points in the animal also be hereditarily determinable? With such complexity of the invisible structure it would not greatly surprise us if we should find amphimixis occurring in all unicellular organisms, and in many of them at a high level of elaboration. These apparently lowly and simple organisms are obviously very far from being the lowliest and simplest, as we shall discover later in a different connexion. But that amphimixis is found as a periodically recurring process even among these, must depend upon the fact that here too the preservation of the best-adapted structure, as well as adaptability to new conditions, requires that the best variants of many different parts of the cell should be brought together, and since the hereditary substance lies in the ids of the nucleus, the union of the ids of two unicellulars will make harmonious and many-sided adaptation materially easier. It will thus give an advantage in the struggle for existence, and we may therefore expect to find that the nuclear substance in all unicellular organism is made up of ids.
[22] Zehnder, Die Entstehung des Lebens, Freiburg-i.-Br., 1899.
Fig. 123. Conjugation of Coccidium proprium, a cellular parasite of the newt (Triton), after Siedlecki. A, a microgamete (Mi) in the act of penetrating the shell of a macrogamete (Ma) through the micropyle. B, the male and the female nuclear constituents are uniting (♂ chr and ♀ chr).
The observations hitherto made do not, however, appear to bear this out, for in the lower Flagellata and Algæ the nuclear substance does indeed consist of chromatin, but—as far as it can be made out—of a compact unarranged mass of it. But even though deeper investigations should succeed in demonstrating chromosomes in many of these, the nucleus must have arisen at some time, and we must assume that it did so through a more intimate union of previously loose aggregates of determinants, which were gradually arranged and bound together by the combining forces (affinities) we have assumed to obtain among them, thus giving rise to the first chromosomes or ids which were complete in themselves. Then came the multiplication of these ids by the process of division, and only then was the state arrived at from which amphimixis, as we now know it, could have arisen, namely, the existence of a considerable number of identical ids, half of which could be exchanged for the identical ids of another individual in conjugation.
But as to our question, In what organisms did amphimixis first arise, and how? there seems, from what we have already learned with regard to the Coccidia, little prospect of our being able to give a definite answer, for if amphimixis occurs even in these lowly organisms, and occurs, too, in the same manner as in the higher unicellular organisms, and not very much more simply than among the highest multicellular organisms, we may conclude that the preliminary stages will now be very difficult or impossible to detect, either because they are extinct, or because they occur only in ultra-microscopic organisms.
Nevertheless there do appear to be preliminary stages, and they are exactly those which we should have assumed if we had been obliged to construct them theoretically.
The first phenomenon of this kind is the mere juxtaposition of two or more unicellular organisms, without the occurrence of fusion. This was probably first observed by Gruber in Amœbæ, and it was theoretically interpreted at a later date by Rhumbler. As many as fifty Amœbæ gather together to form a 'nest,' and remain closely apposed to each other for a fortnight. Although no fusion took place, and there were no visible results of this juxtaposition, it may be concluded that the animals had some sort of attractive effect upon each other, and it may be supposed that some sort of advantage must have been associated with this state of quiet, close apposition against one another. Cytotropism, the mutual attraction of similar cells, which Wilhelm Roux first observed in the segmentation-cells of the frog's egg, seems to occur also in unicellular organisms, and this may help us to understand how a fusion of cell-bodies may have come about.
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.
How far below the Myxomycetes pure plastogamy reaches we do not know; whether it also occurs among non-nucleated organisms (Haeckel's Monera) we cannot tell from experience, since these assumed organisms have not yet been observed with certainty. Perhaps they all lie below the limits of visibility, and then we could never do more than suppose that plastogamic processes occur among them. Logically and purely theoretically we may suppose that amphimixis occurred first between the plasmic bodies of non-nucleated Monera, then between the cell-bodies of true cells, and finally between the nuclei of cells.
Let us hold fast to what we have found to be probable, namely, that the fusion of individually different simple organisms must or may bring about a direct advantage—a stimulation of the metabolism, and at the same time an improvement of the constitution in different directions, and let us go on to the consideration of cell-fusion combined with nuclear fusion, or complete amphimixis. In this something is added which we can recognize as an important advantage, namely, the combination of two hereditary substances, and thus the union of two variation-complexes which, according to our view, is necessary if transformation of species is to take place. In mere plastogamy such a union of two hereditary masses could only take place in Monera, not in nucleated organisms. If then there are really unicellular organisms which exhibit plastogamy without karyogamy (certain Foraminifera), we have a further proof that these processes of plasmic fusion imply direct advantage, which is distinct from the indirect advantage lying in the mingling of two different hereditary contributions, since in these cases of plastogamy there is no demonstrable mingling of hereditary bodies, no karyogamy.
But as soon as karyogamy or nuclear fusion was associated with mere plastogamy, complete amphimixis could never be lost again, because it alone made it possible that there should be harmonious transformation and adaptation in organisms which were becoming ever more complex; the primary effect of the mingling would be more and more transcended, since, without amphimixis, transmutation with harmonious adaptation in all directions would be less and less possible as organisms became more complex in structure. I have already referred to the manifold details in the structure and development of the lowest organisms which make this conclusion appear luminous to us, but we can also infer the necessity for an unceasingly active selection, from a quite different set of facts, namely, from what we know of rudimentary organs in Man.
We may regard Mankind as a species which has its local races and sub-races, but which is fixed in its essential characters, and only fluctuates hither and thither in individual variation in each sub-race, just like any other modern mammal, such as the marmot or the hare. Nevertheless we know that Man, as regards certain fairly numerous parts, is continually and persistently varying in a definite direction. Wiedersheim, in his book On the Structure of Man[23], enumerates a long series of parts and organs of the human body, which are in process of gradual degeneration, and of which it may be predicted that they will disappear from the human structure since they have lost functional significance. Among these dwindling structures are the two last ribs, the eleventh and twelfth, while the thirteenth has already disappeared, and only occurs exceptionally as a small vestige in the adult human being of to-day. The series includes also the seventh cervical rib, the os centrale of the wrist, the wisdom teeth, and the vermiform appendix of the intestine. The last is much larger in many mammals, and represents an important part of the digestive apparatus, but in Man it has dwindled to an unimportant appendage, which is a source of danger when foreign bodies (cherry stones and such like) lodge in it and set up inflammation. The variations in its length warrant us in concluding that it is still in process of degeneration; its average length is about 8½ cm., but it varies from 2 cm. to 23 cm. in length, and in about 25 per cent. of cases a partial or entire closing up of its opening into the intestine may be observed.
[23] Ueber den Bau des Menschen, 2nd ed., Freiburg-i.-Br., 1893. Trans. London, 1896.
Wiedersheim enumerates nearly a hundred parts thus in process of degeneration: this means that nearly a hundred structures in Man are at the present time in process of variation, and this could not be so unless amphimixis were continually mingling the hereditary contributions anew from generation to generation, so that the minus-variations of the parts in question, starting from the germ-plasm in which they arose at one time as chance variations, and confirmed in their direction by means of germinal selection, are gradually being transmitted to all the germ-plasms of the species. We thus see that even in a period of species-life, which we may fairly call a period of constancy, variations of a phyletic kind are continually in process, which could not become general without the co-operation of amphimixis.
Now, we have already seen that personal selection plays no part, or, at least, no important part in such degenerations, because the variations which are here concerned do not usually attain to selection value, but it is just such variations proceeding with infinite slowness that occur in functionally important organs likewise, and in the progressive advance of which personal selection and mutual adaptation probably play a part, so that in this way we can understand why the preservation of amphigony by natural selection must be effected. It is impossible—for obvious reasons—to name particular instances with certainty, as we can do in the case of the rudimentary organs, but even on general considerations we might expect that among the incipient variations of the determinants of the germ-plasm there would be some which were in an ascending direction, and that among these there would be some which, advanced by germinal selection, would go on ascending until they attained selection value. Wiedersheim reckons, for instance, the gradually increasing differentiation of the cortical zone of the human brain among the parts which are still in process of ascending variation, and he is probably right in doing so.
But if variations, so slow as to be unnoticeable, are still of abundant occurrence in Man, we have no reason to doubt that similar processes are going on in other animals; among the higher Vertebrates at least there is hardly a species which does not exhibit regressive variations even now, and in many cases progressive variations also are occurring, although we cannot give definite proofs of this.
The appearance of fixity which most species have is, therefore, illusory; in reality they exhibit a slow flux, gradually setting aside the superfluities they received from their ancestors, perfecting the important parts to more precise adaptation and greater functional capacity, and at the same time endeavouring to maintain all the parts in constant harmony. We can understand that as long as this process of gradual perfecting goes on, amphimixis will not readily be given up. Those that retain it must always, in the long run, have the preference. Moreover, as we have seen, it cannot be given up, when it has existed through æons, because of the power of persistence which the germ-plasm has gradually acquired in the course of such a long hereditary succession. It could only be given up if an advantage decisive as to survival were associated with its abandonment, such as can be actually recognized in most cases of parthenogenesis, among animals at least.
In my opinion this indirect effect of amphimixis, that is, the increasing of the possibilities of adaptation by new combinations of individual variational tendencies, is the main one, while the direct nutritive effect of the two germ-cells upon one another is quite subsidiary. In this opinion I find myself in opposition to the views of many if not most naturalists, who assume that amphimixis has a direct, sometimes, indeed, only a direct effect, and believe that they can prove it by facts.
In support of this position it has been pointed out that allogamy, that is, the mingling of individuals of different ancestry, occurs even among lowly unicellulars, and then higher up among most organisms; but the question has not been asked whether this mutual attraction of the unlike really expresses a primary characteristic of organisms, and may not possibly be a secondary acquisition adapted to ensure the occurrence of amphimixis. If we examine the facts we find that even in the lowly Algæ, such as Pandorina and Ulothrix, only the migratory cells or swarm-spores of different cell-colonies conjugate with one another, but not those of the same lineage, and this phenomenon may be observed in many unicellular plants and animals. We are justified in concluding from this that a fairly large degree of difference between the conjugating gametes secures the best results, whether this result is to be looked for in a 'rejuvenation' or in an increased adaptive capacity; but it is erroneous to regard the stronger attraction between individuals of different descent as a direct outcome of this. To me, at least, it seems to be an adaptive arrangement. The whole of the long and complex phylogenetic history of the sex-cells, the gametes, shows clearly that we have here to deal with a succession of adaptations, and that the degree of attraction which obtains between gametes has gradually been increased and specialized in the course of the phylogeny. I need only briefly recall what we have discussed in a former lecture, that at first the copulating cells were exactly alike in appearance and size, that then one kind of cell became rather larger than the other, and that only gametes which were thus different in size were mutually attractive—the micro-gametes and the macro-gametes, or male and female germ-cells; we have seen that these differences between the two became more and more accentuated, that the female cell continued to grow larger than the male, and to accumulate more and more nutritive material for the building up of the young organism which arises from its union with the male cell, and that the male cells became smaller, but more numerous, as was essential if their chances of finding the often remote female cell were not to disappear altogether. And besides, there are all the innumerable adaptations of the egg-cell to the countless special circumstances which obtain in the different groups, and the innumerable varieties in the form of the sperm-cell, with all its delicate and complicated adaptations to the special conditions under which the egg-cell can be reached and fertilized in this or that group of organisms. Of a truth, he is past helping who does not regard with wonder and admiration the adaptations which have been worked out in this connexion in the course of evolution! But if all these details are adaptations, so is the beginning of the whole process of differentiation; allogamy, the attraction of conjugating cells of different lineage, is not a primary outcome of individual diversity; gametes of different descent did not strongly attract each other of themselves, but they were equipped with a strong power of mutual attraction, because the union of very different individualities was the more advantageous.
This is an important distinction, for the adaptation to allogamy is widely distributed, and its latest manifestations have frequently been misunderstood in the same way as its beginnings. The widespread occurrence of allogamy has been interpreted as evidence in favour of the rejuvenation theory, and the endeavour on Nature's part to secure the union of the unlike has been associated with the hypothetical 'rejuvenating' power of amphimixis, and regarded as a direct and inevitable outcome of this. That this view is erroneous we shall see even more clearly from what follows.
As among unicellular Algæ it is frequently only gametes of different lineage which conjugate, so among animals and plants there are numerous cases in which the union of nearly related gametes is more or less strictly excluded, both by the prevention of self-fertilization (autogamy) in hermaphrodites, or by the prevention of inbreeding, that is, the continued pairing of near relatives. Now all the preventive measures which effect this are of a secondary nature; they are adaptations which result from the advantage involved in the mingling of unrelated germ-plasms, even though it sometimes seems as if they were an outcome of the primary nature of the germ-cells.
The primary result of the mutual chemical influence of the two germ-cells upon one another is—apart from the impulse to development which the centrosphere of the sperm-cell supplies—as far as I see, only the more favourable or the more unfavourable mingling of the biophor- or determinant-variants, and the resulting increase or decrease in adaptive capacity, which leads to the better thriving of the offspring, or conversely to its degeneration. Everything else is secondary and depends upon adaptation, effected in very diverse ways, to secure the most favourable mingling of the germ-plasms for the particular species concerned. Undoubtedly the parental ids united through amphimixis have an effect upon each other, since throughout the building up of the organism of the child the homologous determinants struggle with one another for food, but they do not affect each other in the way that many prominent physiological and medical writers suppose, namely, that the union of the parental germ-plasms sets up a 'formative stimulus' which 'advances' or even 'greatly advances' the process of development in the egg.
Parthenogenetic development goes on just as rapidly, sometimes even more rapidly than that of the fertilized ova of the same species! How can the supposed 'formative stimulus' be so entirely dispensed with in this case?
Of course I am well aware that the two kinds of germ-cells have a strong attraction for each other, and that the protoplasm of the ovum actually exhibits tremulous movement when the spermatozoon penetrates through the micropyle. I myself observed this in the case of the lamprey (Petromyzon) when Calberla instituted his investigations on the fertilization of that animal, but has that anything to do with a formative stimulus? Is it anything more than the result of the chemotactic stimulus exerted by the substance of the ovum upon that of the spermatozoon and conversely? And have we any ground for seeing anything more in this than an adaptation of the sex-cells to the necessity of mutually finding each other out and thereafter combining? Two quite different things are often confused with one another in this connexion: the mutual attraction of the two kinds of sex-cells which tends to secure their union, and the results of this union. A more exact distinction is necessary between the effects and the advantages which allogamy brings in its train and the means by which it is secured in the different species.
If amphimixis really set up a 'formative' stimulus, and if the amount of this was regulated by the differences between the two parental germ-plasms, then parthenogenesis, which implies the entire absence of the mingling of two parental cells, would necessarily be even less advantageous than amphimixis between near relatives; but this is not the case. Continued inbreeding leads in many cases to the degeneration of the descendants, and particularly to lessened fertility and even to complete sterility. Thus in my prolonged breeding experiments with white mice, which were later carried on by G. von Guaita, strict inbreeding, effected throughout twenty-nine generations, resulted in a gradually diminishing fertility, and similar observations have been made by Ritzema Bos and others. But why does not the same thing happen in pure parthenogenesis? My experiments in breeding parthenogenetic Ostracods (Cypris reptans) shows that these crustaceans, in the course of the eighty generations which I have observed till now[24], have lost nothing of their prolific fertility and vital power; and the same is true in free nature of the rose-gall wasp (Rhodites rosæ), which enjoys the greatest fertility notwithstanding its purely parthenogenetic reproduction, the females not infrequently laying a hundred eggs in a single bud. How does it happen that 'the mutual influence of two different hereditary substances which so powerfully promotes individual development' can be here altogether dispensed with? Only because it does not really exist, except in the imagination of my opponents, still influenced by the old dynamic theory of fertilization.
[24] The cultures were begun in 1884 and are still continued (March 6, 1902), still multiplying as abundantly as at the outset. I reckon that there are on an average five generations in a year, which means about eighty generations in sixteen years.
But it may be asked, whence come the injurious results of inbreeding, if not from the union of two nearly related germ-plasms? They certainly do arise from that cause, but it is not through a 'formative stimulus,' too slight in this case, exercising a direct formative chemical effect upon the two hereditary substances, but through the indirect influences exerted by these too similar hereditary contributions during the development of the new individual. Lest it be imagined that I am tilting against windmills, I will refer to one of the numerous examples of the evil effects of inbreeding which have been submitted to me as specially corroborative of the conception of amphimixis as a 'formative stimulus' whose strength depends upon the difference between the germ-substances. The renowned breeder, Nathusius, allowed the progeny of a sow of the large Yorkshire breed, imported from England when with young, to reproduce by inbreeding for three generations. The result was unfavourable, for the young were weakly in constitution and were not prolific. One of the last female animals, for instance, when paired with its own uncle—known to be fertile with sows of a different breed—produced a litter of six, and a second litter of five weakly piglings. But when Nathusius paired the same sow with a boar of a small black breed, which boar had begotten seven to nine young when paired with sows of his own breed, the sow of the large Yorkshire breed produced in the first litter twenty-one and in the second eighteen piglings.
How could this really remarkable difference in the fertility of the sow in question be the result of a formative stimulus, exercised by the sperm-cells of the unrelated boar upon the ova of the female animal? If the progeny of the sow had been more fertile than herself, then we should have been at least logically justified in concluding that this was the case, but it is not intelligible that the egg-cells of this mother sow should be increased twice or three times because they were fertilized by a new kind of sperm as they glided from the ovary. The number of ova which are liberated from the ovary depends in the first instance upon the number of mature ova contained in it; and unless we are to make the highly improbable assumption that the crossing with the strange boar had as an immediate result the maturing of a large number of ova, we must look elsewhere than in the ovary of the animal for the cause of this sudden fertility, possibly in chance circumstances which we are unaware of and which make the ovary occasionally more productive, possibly however in the fact that inbreeding may have brought about various slight structural variations in the animal, and among these some which made the fertilization of the abundantly produced ova by the sperm of the related boar less easy, and caused it to fail more frequently. As will be readily understood, I cannot say anything definite on this point, but we know that very slight variations in the sperm-cell or the ovum may make fertilization difficult, or may even prevent it. I need only remind you of the interesting experiments in hybridization which Pflüger and Born made with Batrachians nearly thirty years ago, which showed that in two nearly related species of frog the ova of the species A were frequently fertilized by the sperms of the species B, but not conversely, the ova of the species B by the sperms of A. This is the case, for instance, with the green edible frog (Rana esculenta), and the brown grass-frog (Rana fusca), and the reason of this dissimilarity in the effectiveness of the sperm lies simply in 'rough mechanical conditions,' in the width of the micropyle of the ovum, and the thickness of the head of the spermatozoon. If each species possesses a micropyle which is exactly wide enough to admit of the passage of the spermatozoon of its own species, another species will only be able to fertilize these eggs if the head of its spermatozoon be not larger than that of the first species. Thus, as experiment has proved, the spermatozoa of Rana fusca fertilize the ova of almost all other related species, for they have the thinnest head and it is at the same time very pointed. In this case, therefore, it depends upon the microscopic structure of the ovum whether fertilization can take place or not, and we can imagine that similar or perhaps other minute variations had taken place in the ova in the case of Nathusius's sow, and that these made it difficult for the sperms of boars of the same family to effect fertilization. These variations may have arisen as a result of the continued inbreeding, because the same ids were constantly being brought together in the fertilized ova, and thus any unfavourable directions of variations which existed were strengthened.
It seems to me that in this way alone can the injurious effect of inbreeding be made intelligible. From both parents identical ids meet in the fertilized ovum, in greater numbers the longer inbreeding continues, for at the maturation of every germ-cell the number of different ids is diminished by a few, and their number must therefore gradually decrease, and it is conceivable that ultimately it may sink to one kind of id, that is, that the germ-plasm may then consist entirely of identical ids. If chance variations of certain determinants in unfavourable directions occur in some of the ids composing the germ-plasm, these are brought together in the offspring from both the maternal and the paternal side, and will occur in an increasing number of ids the longer the inbreeding has gone on, that is, the smaller the number of different ids has become. The unfavourable variation-tendency is therefore persisted in, and its influence upon the development of a new descendant will be the greater the larger the number of identical ids with these unfavourable variations. It is obvious that the crossing of an animal, which is thus, so to speak, degenerating slightly, with a member of an unrelated family must immediately have a good effect upon the descendants, for in this way quite different ids with other variations of their determinants are introduced into the inbred germ-plasm which had become too monotonous.
From this theoretical interpretation of the injurious consequences of inbreeding we may at once infer that not every inbreeding necessarily implies degeneration, for the occurrence of unfavourable variational tendencies in the germ-plasm is presupposed as the starting-point of degeneration, and if these do not exist there can be no degeneration. This harmonizes with the fact that the evil effects of inbreeding are observed to vary greatly in amount, and may not occur at all. But they are greatest in breeds artificially selected by man, which have long been under unnatural, directly influential conditions, and are also removed from the purifying influence of natural selection. In such cases, therefore, there is every probability that diverse unfavourable variational tendencies in the determinants will occur.
But how are we to understand the fact that pure parthenogenesis may last through innumerable generations, and yet no degeneration sets in? I believe very simply. In this case too, the same ids which were peculiar to the mother of the race are contained in the descendants, but they do not diminish in number, for in pure and normal parthenogenesis, such as that of Cypris reptans, the second maturation-division of the ovum does not take place, and this is precisely the nuclear division which effects reduction. In addition, the introduction of identical ids, which must take place in the case of inbreeding at every amphimixis, does not occur, and, what is certainly of great importance, all these cases are old species, living under natural conditions—the same conditions under which they lived as amphigonous species, and not newly formed breeds under artificial conditions, as has probably always been the case in the experiments in inbreeding.
It is true that even in old species, living in a state of nature, unfavourable variations may arise in the germ-plasm, and may go on increasing during purely parthenogenetic multiplication, for the ids with unfavourably varying determinants will no longer be set aside by means of reducing division. But those individuals in which the unfavourable variational tendency increases until it has attained selection-value will be subject to selection and will be gradually eliminated; indeed, the weeding out of the inferior individuals will be more drastic here than where amphigony obtains, because in this case all the offspring of one mother are nearly alike, so that the whole progeny is exterminated if the mother varies unfavourably.
On the other hand, a transformation in a favourable direction, an adaptation to new conditions of life, as far at least as that implies the simultaneous variation and harmonious co-adaptation of many parts, cannot, as far as I can see, be effected in the course of purely parthenogenetic reproduction, nor can a degeneration of complicated parts which have become superfluous. For both these changes, in my opinion, require that the ids of the germ-plasm should be frequently mingled afresh, since apart from this there cannot be a harmonious readjustment of complicated structures, nor can a uniform degeneration affecting all parts set in. As an example of this last case we may take that organ which became functionless in the purely parthenogenetic species of Ostracods when amphigonous reproduction was given up—the sperm-pocket or receptaculum of the female. All these species still possess an unaltered receptaculum seminis, a large pear-shaped bladder with a long, narrow, spirally coiled entrance-duct, very well adapted for allowing the enormous spermatozoa of the males to make their way in singly, and to arrange themselves within the receptacle side by side in the most beautiful order, like a long ribbon, and finally to migrate out again singly to fertilize the liberated ova. In Cypris reptans and several other species, however, no males have been found in any of the places which have been carefully searched, and the receptaculum of the female is always found to be empty. Nevertheless it shows no hint of degeneration. It is possible enough that, as in Apus cancriformis, which is of similar habit, the males have become extinct in most colonies of these species, but that nevertheless they do occur here and there from time to time in the area inhabited by the species, and if this should prove to be the case, it would confirm the conclusion, which is very probable on other grounds, that the pure parthenogenesis of these species has not existed in most of their habitats for a long time, speaking phylogenetically. For this reason we must not over-estimate the significance of the complete persistence of the receptaculum even with exclusively parthenogenetic reproduction. It proves, however, that degeneration of a superfluous organ does not necessarily set in even after hundreds of generations, and in this fact there is certainly a corroboration of the view that it is 'chance' germinal variations which give the impulse to degeneration. These first induce a downgrade variation through germinal selection, and this, if it concerns an organ of no importance to the survival of the species, is not hindered in its progress by personal selection. Whether degeneration of the receptaculum would have occurred in these parthenogenetic species if they had retained even a periodic sexual reproduction, as is actually the case in the generations of the alternately parthenogenetic and sexually reproducing Aphides, we cannot decide, since we know nothing in either case as to the length of time that parthenogenesis has prevailed among them, nor have we any method of computing the number of generations that must elapse before a superfluous organ begins to vacillate. We only know that the parthenogenetic generations of Aphides no longer possess a receptaculum, while other forms with alternating bi-sexual and parthenogenetic modes of reproduction, which are in this respect possibly more modern, e. g. some of the gall-wasps, possess one similar to that of the Ostracods.
Fig. 79 (repeated). The two maturation divisions of the 'drone eggs' (unfertilized eggs) of the bee, after Petrunkewitsch. Rsp 1, the first directive spindle. K 1 and K 2, the two daughter-nuclei of the same. Rsp 2, the second directive spindle. K 3 and K 4, the two daughter-nuclei. In the next stage K 2 and K 3 unite to form the primitive sex-nucleus. Highly magnified.
I must refer to one other case of parthenogenesis, since it has been hitherto regarded as a formidable puzzle for the germ-plasm theory, and has only recently found its solution, I mean the facultative parthenogenesis of the queen-bee. As the 'male' eggs of the bee remain unfertilized, and yet undergo two reducing divisions, which must diminish the number of ids in the ovum-nucleus by a half, the number of ids in the germ-plasm of the bee must be steadily decreasing, and this state of things has therefore been regarded by some English biologists as convincing evidence of the untenability of the conception of ids and of the whole germ-plasm theory. Apparently, indeed, it is contradictory to the theory, and we must inquire whether the contradiction is merely an apparent one, disappearing when the facts are more precisely known. It was mainly on this ground that I instituted the researches carried out by Dr. Petrunkewitsch, the results of which I have already in part communicated in a former lecture. These results confirmed the previous conclusions that the 'male' eggs of the queen-bee remain unfertilized, that two reducing divisions occur, and that in consequence the ovum-nucleus only contains half the normal number of chromosomes. That these increase again by division to the normal number does not save the theory, for only identical ids can arise in this way, while the significance of the multiplicity of the ids lies mainly in their difference. The halving of the number of ids in each 'male' ovum would necessarily lead, if not to a permanent diminution in the number of ids, at least to a monotony of the germ-plasm, since the number of different ids would be steadily decreasing and the number of identical ids as steadily increasing. This too would be a contradiction of the theory. But Dr. Petrunkewitsch's investigations have shown that, of the four nuclei which are formed by the two reducing divisions, the two middle ones (Fig. 79, K 2 and K 3) recombine with one another, and fuse into a single nucleus, and that from this copulation-nucleus in the course of development the primitive germ-cells of the embryo arise. Now all the ids which were originally present in the nucleus of the immature ovum may be reunited in this 'polar copulation-nucleus' if the two nuclei K 2 and K 3 turned towards each other in Fig. 79 contain different ids. That this is the case cannot of course be seen from the ids themselves, but it seems to me extremely probable, since it is dissimilar poles of the two nuclear spindles which here unite, namely, the lower pole (daughter-nucleus) of the upper spindle and the upper pole of the lower spindle. In the first directive or polar spindle there lay thirty-two chromosomes, which had increased by duplication from sixteen, and of these sixteen passed over into the first polar nucleus, while sixteen formed the basis of the second directive spindle. These two sets of sixteen chromosomes must have been quite similar, since the two sets arose by division of the sixteen mother-chromosomes. Let us call the chromosomes a, b, c, d-q, then similar sets of chromosomes must have been contained in the two nuclear spindle figures depicted in Fig. 79 at the beginning of the division, and eight of these went to each daughter-nucleus. Now, if a-k migrated to the upper pole of the spindle and l-q to the lower pole, then the union of K 2 with K 3 would bring together again all the ids that had before been present. In consideration of this I predicted to Dr. Petrunkewitsch that this copulation-product might be the basis of the formation of the germ-cells in the drone-bee, and his painstaking and difficult researches have confirmed this prediction, strange though it may seem, that the male germ-cells have a different origin from the female germ-cells. But this discovery gives a strong support to the germ-plasm theory. It may, of course, be objected that the assumed regular distribution of the ids in the two daughter-nuclei cannot be proved, but we know already that this dividing apparatus does very exact work, and we are at liberty to assume it in an even higher degree. Moreover, what other interpretation of the unexpected development of the germ-cells discovered by Petrunkewitsch could be given if this had to be rejected? A clearer proof of the individual differences of ids and of their essential importance could not be desired, than lies in the fact that in the 'male' eggs of the queen-bee a different and novel mode of germ-cell formation is instituted, after half the ids have been irrecoverably withdrawn from the ovum-nucleus. We see from this that for individual development a duplication of individual ids may suffice, but that for the further development of the species a retention of the diversity of the ids is important.