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.