LECTURE XXX
INBREEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION,
AND THEIR CONSEQUENCES
The separation of the sexes exists even among the Protozoa—Conditions determining the occurrence of Hermaphroditism—Tape-worms, Cirrhipeds—Primordial males—Advantages of parthenogenesis—Alternation with bi-sexual generations—In Gall-wasps—In Aphides—Cross-fertilization secured in plants—Self-fertilization is avoided whenever possible—The mechanism of fertilization and the mingling of germ-plasms must be clearly distinguished from one another—Cases of persistent self-fertilization—The effects of inbreeding compared with those of parthenogenesis—The effect of purely asexual reproduction—In sea-wracks—In lichens and fungi—In cultivated plants—Degeneration of the sex-organs—Summary.
We have seen that continued inbreeding must make the germ-plasm monotonous, and therefore unplastic as regards the requirements of adaptation. Accordingly, we found that the gametes of many unicellulars are so constituted that they only possess a power of attraction for gametes of a different lineage, not for those of their own stock. Among multicellular organisms the most intense mode of inbreeding is to be found in the uninterrupted self-fertilization of hermaphrodites: in such cases the monotony of the germ-plasm must reach extreme expression more readily than in the case of ordinary inbreeding. We can thus understand why, in the scale of organisms, there is such an early occurrence of gonochorism, the separation of the species into male and female individuals. Even among unicellular plants or Protophytes this occurs occasionally, as it does in the Vorticellids among Infusorians.
In the Metazoa and Metaphyta the separation of the sexes finds emphatic expression; it is absent from no important group, and in many, such as, for instance, among the Vertebrates, it has become the absolutely normal condition, with hardly any exception. But in many divisions of the animal and plant kingdoms hermaphroditism also plays an important part, as, for instance, in terrestrial snails and in flowering plants.
Obviously the sexual adaptations of a species are definitely related to the conditions of its life, and, though Nature's endeavour to prevent inbreeding and to secure cross-fertilization is evidenced by the occurrence of separate sexes in such a multitude of forms yet in many cases gonochorism has been relinquished, and always where this was necessitated by the conditions of life to which the group concerned was subject. In such a case inbreeding is regulated as far as possible, for instance, by an arrangement which ensures that individuals shall be crossed at least from time to time. But cases of exclusive and constant self-fertilization do also seem to occur, and even these may be brought into harmony with our conception, according to which cross-fertilization is an advantage, but only an advantage which must be weighed against others, and which may eventually be given up in favour of greater advantages. This occurrence of persistent autogamy can no more be reconciled with the rejuvenation theory than can continuous parthenogenesis, because, according to this theory, the mingling of different individuals is a sine qua non, for the continued life of the species.
It is impossible for me here to discuss in detail all the deviations from pure gonochorism or bi-sexuality which occur in nature, but I must at least attempt to take a general survey, and to arrange the chief phenomena of these various modes of 'sexual reproduction' in an orderly scheme. I must take a survey of both plants and animals, but I shall give the precedence to animals, as being to me more familiar ground.
Where do we find, in the animal kingdom, that Nature has departed from gonochorism, from the separation of the sexes, and for what reasons was this departure necessary? And further, what means does Nature take to compensate for this renunciation of the simplest method of securing the continual cross-fertilization of individuals?
Let us glance over the animal kingdom with special reference to these questions: we find that hermaphroditism prevails chiefly among species which at maturity have lost their power of free locomotion, and have become sedentary, such as oysters, barnacles among Crustaceans, the Bryozoa, and the sea-squirts (Ascidians) which are fixed to the rocks at the bottom of the sea. For forms such as these it must often have been advantageous that each individual could function both as male and as female, especially when it was capable of self-fertilization, since individuals which settled down singly, or in very small numbers together, would not be lost as regards the persistence of the species. The continuance of the species is thus better secured than it would be by separation of the sexes, because in the latter case it might frequently have happened that the animals which had settled beside each other by chance were of the same sex, and would therefore remain unfertile. But many of these species do not fertilize themselves, but fertilize each other mutually; and this, too, carries a great advantage with it, because in sedentary animals the sperms will fertilize twice as many individuals, if each contains eggs, than if half were exclusively male. It is thus to some extent an economy of sperms, but at the same time also of ova, which is effected by hermaphroditism: the result is that these valuable products are wasted as little as possible. On this account we find that not only sedentary, but also sluggish, slow-moving animals are equipped with male and female organs of reproduction, as, for instance, all our terrestrial snails. They fertilize each other mutually: when two meet it is always as males and females, and notwithstanding the sluggishness of movement, it is not likely to happen that a snail does not attain to reproduction because it has not found a mate. The same is true of the earthworms, which are likewise not adapted for making long journeys in search of the opposite sex; they, and the leeches also, function as male and female simultaneously, while their nearest relatives, the marine Chætopods, are of separate sexes, which may be associated with their much greater power of free movement in the water.
In these cases self-fertilization is often absolutely excluded; it may be physically impossible, and hermaphroditism therefore secures cross-fertilization in such cases just as effectively as if the sexes were separate. Similarly, in many hermaphrodite flowers, as we have already seen, the pollen is so constituted and so placed within the flower that it cannot of itself make its way to the stigma. In oysters, for instance, the young animal is male, and liberates into the water an enormous quantity of minute spermatozoa, and therewith fertilizes the older individuals, functioning only as females, which have grown upon the same bank. At a later stage of its development the oyster which was male becomes female, and produces only ova. This state of affairs, of which I shall shortly mention another case, has been called temporary hermaphroditism. In this case not only is self-fertilization excluded, but close inbreeding also, since it is always a young generation functioning simultaneously as males that mingles with an older generation which has become female.