If we took our stand, however, upon the facts before cited we could conclude without hesitation that the celebrated thesis of Weismann regarding the immortality of the ciliate Infusoria had been overthrown. But the phenomena are not presented with this simplicity. When the vitality of the Infusoria has become weakened by a considerable number of agamic reproductions, and the animalcule is upon the point of dying a natural death, a new biological phenomenon can intervene, rejuvenating the animal and rendering it capable of reproducing itself anew for a long series of generations. That phenomenon is fecundation.
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In our work upon Micro-Organisms we have spoken at length of the material process of fecundation in ciliate Infusoria, and of the phenomena preliminary to it, following as our guide the observations of Balbiani, Gruber, Bütschli, and Engelmann. It will be necessary to recur here to that subject and to supplement our preceding exposition with some important details. Moreover, recent researches, added to other older ones, afford us interesting information with regard to the conditions and determining causes of conjugation and also of the significance of fecundation itself.
We have seen above that according to M. Balbiani an active period of agamic bipartition in Infusoria can terminate in a period of conjugation; a circumstance which produces in effect a cyclical alternation between agamic generations and a sex-generation. The very word cycle is used in the observations of M. Balbiani. M. Maupas elevated this observation of M. Balbiani to the rank of a method; using, in order to procure the great number of coitions necessary for his investigations, the following process. He placed the Infusoria in water in which he had produced a putrid fermentation. The Infusoria, thanks to the abundance of the nutriment developed in great numbers. While thus swarming they were lifted out with a drop of water, which was kept upon the stage in a moist chamber. The Infusoria there continued to grow larger and multiply; but by reason of their great numbers it was not long before they exhausted the food brought with them in the drop of water. When the last remains of their nutriment had disappeared they were seen in the majority of cases to seek each other and to copulate.
According to M. Maupas, it is not solely the weakness produced by a series of bipartitions, but, in addition to that and more particularly, the scarcity of food, that excites in the ciliate Infusoria the conjugal appetite. The epidemics of conjugation of which the authors speak, are not otherwise explainable. M. Maupas even says, that when a number of pairs are about to copulate, it is only necessary to give them food to put an end to their conjugation. Scarcity, that author further remarks, ought evidently not to modify in any essential the internal organic state of the Infusoria in question; no more indeed than the opposed condition, that is, an abundance of rich food (page 403 of his memoir). But in the first case they copulate without any ado; in the second, they refuse to do so entirely. Rich alimentation deadens the conjugal appetite; fasting, on the contrary awakens and excites it. There exists moreover, according to the author last mentioned, in ciliate Infusoria, a particular period beyond which fecund coitions cannot take place. It is what he calls the period of karyogamic maturity. Thus, in Leucophrys, for example, fecund coitions are observed to take place only after the three-hundredth generation. Before that time the Infusoria may be placed in all the other conditions favorable to copulation, without being seen to contract a single union. On the other hand, beyond that time, a period extends in which numerous coitions are obtained. Although, indeed, the cyclical alternation of agamic generations and copulations is indisputable, further researches are still necessary to obtain a thorough knowledge of the extent of these cycles. It is certain that their duration varies in the different species, and perhaps, in conditions as yet imperfectly known, may in any one species be considerably abridged.
We are now come to the preliminaries of copulation. We have described them in our work, making use of the observations of Balbiani, Gruber, and of Engelmann, some of which we found confirmed by Bütschli. M. Maupas, who has recently again taken up this question, believes he has discovered in his predecessors, or rather in the observations of M. Balbiani, grave errors. I shall transcribe the passage in question: "When a numerous group of Infusoria of the same species are found in the conditions that determine copulation, these animalcula abandon themselves to certain movements, and exhibit an agitation the significance of which has been much exaggerated. Balbiani, who in fact always seeks analogies with the higher animals, has given us an animated description of these movements, to which a poetical imagination has contributed at least as much as exact and scientific observation. This description has met with a most favorable reception among certain philosophers and psychologists who have taken up with it in the belief that they could thereby reveal in microzoans the rudiments of the instincts and psychic faculties of higher-organized beings. As there is very much inexactitude and exaggeration in all that, it is time to calm this enthusiasm and to refer the facts and their explanation to some more exact criterion." (Page 413.)
I believe it useless to occupy my time in dealing with the aggressive tone that this author has seen fit to assume towards me, and which seems to be habitual with him when he criticises the works of people with whom he does not agree. I shall carefully examine his observations and seek to derive from them some profit, to the improvement and correction of my work upon the Psychic Life of Micro-Organisms, if it is true that I have committed the grave errors of which he speaks. Besides, the question of the preliminaries of copulation is so interesting in itself that I have no fear of turning to it a second time.
It is necessary, here, clearly to distinguish two things: the facts and their explanations. In that which concerns observed facts, the errors that M. Maupas endeavors to point out in the descriptions of M. Balbiani appear to me to be capable of a reduction to a matter of so little significance—admitting that it comes at all from error—that if I had not been apprised of it, I should have regarded the researches of the first author as a confirmation in most details of those of the second. It is to be observed, in fact, that M. Maupas gives almost the same description that Balbiani does of the movements of Paramæcium aurelia. "I have followed animals of that species a number of times," he says, "during the preparations for copulation. They exhibit at that moment a very great agitation. They are seen to go and come, rapidly changing their direction. They approach and throw themselves against their congeners, halt in front of them, feel them an instant with their cilia, then leave them, assume the most varied positions, and, finally, when two individuals equally ready for union chance to meet each other, they face about by their anterior extremities so that the two bodies come together and join, with the exception of the posterior extremities, along their whole extent; the union is thus definitively effected."
Up to this point, let it be observed, our author's description is but a paraphrase of that of M. Balbiani, which we have given on page 69 of our work; and a simple comparison of the two suffices to prove this. The divergences of fact extend, as it seems to me, to the two following points only: The duration of the preliminaries, and the existence of an epidemic of copulation. M. Maupas thinks that the movements in question never last very long, at the most a quarter of or half an hour among individuals that have arrived at karyogamic maturity. Whereas M. Balbiani has observed these same movements last for several days. I do not know which of these two observations is the more exact; in fact, I do not think it necessary to choose between them, since both may be exact, the duration of the phenomenon generally being dependent upon conditions subject to great change, while M. Maupas himself remarks that the ciliate Infusoria in the variability of all their biological phenomena are veritable thermometers of a very great sensibility. However that may be, whether the movements that precede copulation in Paramæcium last a quarter of an hour, half an hour, or several days, that fact does not change their real character. The second divergence relates to the epidemics of copulation in the case of Paramæcium aurelia; observed by M. Balbiani and denied by M. Maupas. "All the individuals of a group," says M. Maupas, "are never found simultaneously in this condition. Hence the tentative preliminaries of copulation, that fail in their object and end in the individuals going to seek elsewhere another partner." I confess, I do not understand this statement, involving, as it does, M. Maupas in a contradiction; for two pages before this he speaks of the mode of the appearance of copulation as in the epidemic form. All observers, he says, that have occupied themselves with this phenomenon, state that it is suddenly developed in the little aquariums in which the animals are contained, and very rapidly becomes general (page 41).
To this then the divergences of fact are reduced—a matter entirely insignificant; and I believe it useless to dwell upon it longer. The question of interpretation remains. I shall also say a few words with reference to this, although the disagreement is at bottom not much more serious.