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.
It is quite otherwise with parasites which live singly within the body of a host: for these it was indispensably necessary that they should not only produce both kinds of germ-cells, but that they should unite the two kinds in fertilization, and they therefore possess the power of self-fertilization. Thus, in the urinary bladder of the frog, there occurs a flat-worm (Polystomum integerrimum) which possesses special organs for pairing with another individual, but which is also capable of self-fertilization when, as frequently occurs, it has no companion in its place of abode. But this self-fertilization is always liable to be interrupted by cross-fertilization, for not infrequently there are two, three, or even four such parasites within the bladder of a single frog.
In the tape-worms, too, cross-fertilization is not excluded, for there are often two or more of these animals together in the intestine of a host at the same time. But even where there is only one, self-fertilization on the part of the joints, that is, the sexual individuals, is prevented, and by the same device, metaphorically speaking, as in the case of the oyster, for in each joint the male elements mature first and the female elements afterwards. In certain parasitic Isopods of the genus Anilocra and related forms close inbreeding is prevented in the same way—by a difference in the period at which the two sets of gonads in the hermaphrodite individual become mature (dichogamy).
This is secured in a different way in Crustaceans which have grown to maturity in a sedentary state, like the Cirrhipeds. These animals, known as 'acorn-shells' and 'barnacles,' are sedentary, sometimes on rocks and stones, sometimes on a movable object, the keel of a ship, floating pieces of wood, cork, or cane, or sometimes attached to turtles or whales, and although they generally occur in great numbers together, they are probably only able to fertilize each other occasionally, and are therefore essentially dependent upon self-fertilization. But Charles Darwin discovered long ago that many of them, notwithstanding their hermaphroditism, have males which are small, dwarf-like, and very mobile organisms, destined only for a very brief life. These seemed quite superfluous in association with hermaphrodite animals, and they have therefore long been regarded as vestigial males, as the last remnant, so to speak, of a past stage of the modern Cirrhipeds, in which the sexes were separate. It is obvious, however, that we must now attribute to them a deeper significance, for these so-called 'primordial males,' although extremely transitory creatures without mouth or intestine, represent a means of securing the cross-fertilization of the species. What importance nature attaches to their preservation is shown especially by the parasitic Cirrhipeds which have been so carefully studied by Fritz Müller and Yves Delage—those sac-like Rhizocephalidæ or root-crustaceans which are altogether disfigured by parasitism. The fully developed animals are hermaphrodite and live partly in, partly upon crabs and hermit-crabs (Fig. 112, C, Sacc). These hermaphrodites indeed fertilize themselves, but in their youth they are of distinct sexes, and the females are so constituted that they lay eggs for the first time just when the males of the current year are appearing. Thus the first batch of eggs liberated by the females are fertilized by the minute free-swimming 'primordial males,' but after that the females themselves develop testes, and then fertilize themselves; the males die very soon after copulation, and only appear the following year in a new generation. They are therefore far from being mere historic reminiscences, vestiges of the early history of the modern species, for they are the instruments of a regular cross-fertilization of the species, and therefore of a constant mingling of new ids in the germ-plasm. This is not the place to discuss the marvellous life-history of these parasites in detail; I can only say that when we inquire into the whole story, and appreciate the difficulties associated with the persistence of these 'primordial males,' we can no longer doubt that crossing is an indispensable feature of amphimixis—a feature which must at least occasionally occur if amphimixis is to retain its significance. This is shown, it seems to me, especially by these numerous instances of what we may call compulsory retention of ephemeral males in hermaphrodite, self-fertilizing animals; it follows also from the theory, for with continued self-fertilization all the ids in the germ-plasm of an individual would tend to become identical, and the mingling of two germ-plasms which contained identical ids would, at least according to the germ-plasm theory, have no meaning at all.
Fig. 112 (repeated). Development of the parasitic Crustacean Sacculina carcini, after Delage. A, Nauplius stage. Au, eye. I, II, III, the three pairs of appendages. B, Cypris stage. VI-XI, the swimming appendages. C, mature animal (Sacc), attached to its host, the shore-crab (Carcinus mænas), with a feltwork of fine root-processes enveloping the crab's viscera. S, stalk. Sacc, body of the parasite. oe, aperture of the brood-cavity. Abd, abdomen of the crab with the anus (a).
Thus we see that in the animal kingdom hermaphroditism is always associated with cross-fertilization in some way or other, even though the latter may occur rarely, being usually periodically interpolated, and thus bringing new ids into the germ-plasm which is rapidly becoming monotonous or uniform. Adaptations quite analogous to these are found in relation to parthenogenesis, and it will repay us to give a brief summary of these.
Parthenogenesis effects a very considerable increase in the fertility of a species, and in this increase the reason for its introduction among natural phenomena obviously lies. By the occurrence of parthenogenesis, the number of ova produced by a particular colony of animals may be doubled, because each individual is a female, and as the multiplication increases in geometrical ratio a few parthenogenetic generations result in a number of descendants enormously in excess of those produced by bi-sexual reproduction. We can therefore understand why parthenogenesis should obtain among animals whose conditions of life are favourable only for a short time, and are then uncertain and dangerous for a long period. This is the case with the water-fleas, the Daphnids (see Figs. [57] and [58]), whose habitats—pools, ponds, and marshes—often dry up altogether in summer, or freeze in winter, so that it becomes almost if not quite impossible for the colonies to go on living, and the preservation of the species can only be secured by the production of hard-shelled 'lasting' eggs, which sink to the bottom, dry up in the mud, or become frozen, or at least remain latent in a sort of slumber. As soon as the favourable conditions reappear, young animals which emerge from the eggs are all females and reproduce parthenogenetically, so that after a few days there is a numerous progeny swimming freely about, which in their turn are all females, and reproduce after the same manner. In many Daphnids this goes on for a series of generations, and there thus arises an enormous number of animals, which may fill a marsh so densely that, by drawing a fine net a few times through the water, one can draw out a veritable animal soup. In our ponds and lakes these little Crustaceans form the fundamental food of numerous fishes. But notwithstanding the enormous havoc wrought among them by enemies, large numbers remain at the end of a favourable season, and these produce the lasting eggs, after fertilization. For shortly before the end of the season males appear among the progeny of the hitherto purely parthenogenetic females. Although each female will only produce a few of these 'lasting' eggs, which require fertilization and are richly supplied with yolk, the whole number in each colony is a very large one, because the number of individuals is very large; and it must be so, since the eggs, though secure against cold and desiccation, are very imperfectly protected against the numerous enemies which may do them injury.
Of course the number of individuals which form a colony may vary greatly in the different species, and the same is true of the number of parthenogenetic generations which precede the bi-sexual generation. I have already shown in detail that this depends precisely on the average duration of the favourable conditions, so that, for instance, a species which lives in large lake-basins will produce many purely parthenogenetic generations before the bi-sexual one, which only appears towards autumn, while species which live in quickly-drying pools have only a few parthenogenetic generations, and the true puddle-dwellers give rise to males and sexual females along with the parthenogenetic females as early as the second generation.
We thus find in the Daphnids an alternation, regulated and made normal by natural selection, of purely parthenogenetic with bi-sexual generations, and the result is that the uniformity of the germ-plasm, which is the necessary consequence of pure parthenogenesis, is interrupted after a longer or shorter series of generations by the occurrence of amphimixis. That the number of parthenogenetic generations may be so varied, though with a definite norm for each species, indicates again that amphimixis is not an absolute condition of the maintenance of life, not an indispensable rejuvenation, designed to counteract the exhaustion of vital force—whether this be meant in a transcendental sense or otherwise—but that it is an important advantage calculated to keep the species at its highest level, and that its influence appears whether it occurs in the species regularly, or frequently, or only rarely.
This kind of alternation of generations, that is, the alternation between unisexual (female) and bi-sexual generations, has been called heterogony. In the Daphnids, certainly, a difference in form between the parthenogenetic and the bi-sexual generation does not exist, for the same females which produce eggs requiring fertilization can also produce parthenogenetic ova, although these are very different from each other, as we have already seen. The difference between generations, therefore, does not lie in their structure, but in their tendency to parthenogenetic or to amphigonous reproduction, and in the absence or presence of male individuals. There are, however, other cases of alternation of generations in which the different generations diverge from each other in structure. One of the most remarkable of these is that of the gall-wasps (Cynipidæ). In many of these little Hymenoptera, which form galls on leaves, blossoms, buds, and roots, especially of the oak, two generations occur annually, one in summer, the other in early spring, or even in the middle of winter. The latter consists of females only and reproduces parthenogenetically. We can readily understand this from the point of view of adaptation to particular conditions, since the young wasps which emerge from their galls in winter, or in the middle of a raw spring, are exposed to many dangers and are terribly decimated before they can succeed in laying their eggs in the proper place on the plant. Moreover, much precious time would be lost by the mutual search of the sexes for each other,—a search which would often be entirely without result. Thus the wingless female of Biorhiza renum (Fig. 124, A), which is not unlike a plump ant, attempts, without taking food, and often interrupted by a spell of cold or a snowstorm, to reach a neighbouring oak-shrub, creeps up on it, and lays its eggs in the heart of a winter bud, whose hard protecting scales it laboriously perforates by means of its short, thick, sharp ovipositor.
Fig. 124. Alternation of generations in a Gall-wasp. A, winter generation (Biorhiza renum). B and C, summer generations (Trigonaspis crustalis). B, male. C, female. After Adler.
After it has succeeded in sinking its ovipositor into the heart of the bud, it goes on working for hours, piercing the delicate tissue with a multitude of fine canals, one close beside the other, and then deposits an egg in each of these. The whole detailed piece of work requires, according to Adler, uninterrupted active exertion for about three days, even though in the end only two buds may be filled with eggs. If at every egg-laying the arrival of a male had to be waited for, an even larger number of females would fall victims to the unfavourable weather and other dangers, while at the same time the number of emerging females could be only half as large as it is. It is obvious that in this case parthenogenesis is of very great advantage.
In summer the climatic conditions are incomparably more favourable for the gall-wasps, and accordingly we find that the summer generation is bi-sexual, but, strangely enough, is so different from the winter generation that the relationship of the two forms was for a long time overlooked. The antennæ, the legs, and particularly the ovipositor, the whole shape of the animal, its size, the length of the abdomen, the structure of the thorax, and many other points are so different that as long as the structural features afforded the only criterion of relationship, the systematists quite naturally placed the winter and summer forms in different genera. It was only when Dr. H. Adler succeeded in breeding the one form from the other that people were convinced that such marked differences in structure could be found within the same life-cycle.
Fig. 125. The two kinds of
Galls formed by the species. A,
the many-chambered galls produced
by the parthenogenetic
winter form, Biorhiza renum. B,
those produced on oak-leaves by
Trigonaspis crustalis, the bi-sexual
form. After Adler.
But we see here quite clearly why the two generations had to become so different; simply because the winter generation had to adapt itself to different conditions from the summer generation, above all as to the laying of its eggs within the tissues of a plant of a different constitution. In our example, the winter form Biorhiza renum pierces the terminal buds of the oak, and lays in each of them a large number of eggs, sometimes as many as 300, so that a very large gall is formed, in which a great many larvæ can find food, and grow on to the pupa-stage. From this spongy gall, something like an inverted onion in shape, and about the size of a walnut (Fig. 125, A), there emerge in July the slender, delicately formed male and female gall-wasps which were long known as Trigonaspis crustalis. Both males and females are winged, and fly rapidly about in the air (Fig. 124, B and C). The sexes pair, and the females lay their eggs in the cell-layers on the under side of an oak-leaf, on which arise small, wart-like, kidney-shaped galls (Fig. 125, B) which fall to the ground in autumn, and from which there emerge, in the middle of winter, the plump, wingless females, to which, as we have already seen, the name Biorhiza renum was given.
One generation, therefore, lays its eggs in the parenchyma of tender leaves, and has only to pierce through a thin layer of plant-tissue, while the other must penetrate deep down into the hard winter bud, to be able to deposit its eggs in the proper place, and we therefore find that in the two kinds of female the ovipositor differs in length, thickness, and general structure, and so also does the whole complex apparatus by which the ovipositor is moved. But these differences are associated with the form of the abdomen, in which the ovipositor lies, and with the strength and shape of the legs, which must be shorter and stronger when the boring has to be performed through a hard plant-tissue or to a considerable depth. We can readily understand how numerous must be the secondary variations which a transformation of the ovipositor brings in its train when we compare the ovipositor apparatus in the two generations of one of these species (Fig. 126).
Fig. 126. Ovipositor and ovum of the two
generations of the same species of Gall-wasp.
A, those of the winter form, Neuroterus læviusculus.
B, those of the summer-form, Spathegaster albipes.
st, ovipositor. ei, ovum. Similarly magnified.
After Adler.
Figure 126 shows the ovipositor of another gall-wasp, of which the winter form, Neuroterus læviusculus, also perforates the hard winter buds of the oak, while the summer form, Spathegaster albipes, lays its eggs in the tender young leaves of the same tree. The ovipositor of the former is thin and long, that of the latter short and strong (Fig. 126, A and B), and corresponding also to the depth at which the egg must be sunk, or, so to speak, sown in the tissue of the plant, the egg of the summer generation differs from that of the winter generation by having a much shorter stalk (Fig. 126, ei). These little wasps thus afford a beautiful example of the way in which even marked changes in the conditions of life of a generation may be associated with transformations in bodily structure, and we understand how it was possible that by means of processes of selection the generations which alternate periodically in the year should come to diverge very considerably in structure. The example may also serve to illustrate how diverse are the harmonious co-adaptations which such transformations require, and how necessary, therefore, the continual re-combination of the ids of the germ-plasm by means of amphimixis must be. We understand why bi-sexual reproduction was only abandoned in one generation, and that the one in which parthenogenesis was of considerable advantage. But such transformations must have come about with extreme slowness, since they were the result of climatic changes which only come about very gradually. We thus come again to the same conclusion to which we were led by our study of vestigial organs in Man, that numerous species which appear to be at a standstill are continually working towards their own improvement. But for this amphimixis is essential; consequently the descendants which have arisen through amphimixis, and whose ancestors have arisen in the same way, have an advantage over those of parthenogenetic origin. On the whole, at least, this must be so; in special cases it may be otherwise, namely, when the advantage offered by parthenogenesis in respect to the maintenance of the species preponderates over the advantage which amphimixis implies as regards possibilities of transformation.
As far as we have seen from the case of the gall-wasps, the absence of amphimixis in every second generation implies no disadvantage in regard to the capability for transformation which the species exhibits. As to whether any disadvantage would ensue if the number of parthenogenetic generations in the life-cycle were greater we can only guess, since no case is known which enables us to decide this point, pro or con, with any certainty. The heterogony of the plant-lice, the Aphides, and their relatives might be cited as against the probability, for in this case a long series of parthenogenetic generations often alternates with a single bi-sexual one, but the difference in structure is not so great in this case, although it does exist, and moreover we can quite well assume that the adaptation to parthenogenesis was effected at the beginning of heterogony, when it still consisted of a cycle of only two generations, and that further virgin generations were interpolated subsequently.
This assumption is supported by the fact that in some species of our indigenous Ostracods, in Cypris vidua and Candona candens, in contrast to the Daphnids, several bi-sexual generations alternate with one parthenogenetic generation. But in this case again there is no difference whatever in the structure of the two generations, the parthenogenetic generation being distinguished from the bi-sexual generation simply by the absence of males.
The alternation of generations in the plant-lice is particularly instructive, because it emphatically indicates how much Nature is concerned with the retention of amphimixis, and how little mere multiplication has to do with this. This is especially striking in the case of the bark-lice; for instance, in their notorious representative, the vine-pest, Phylloxera vastatrix.
Fig. 127. Life-cycle of the Vine-pest (Phylloxera vastatrix), after Leuckart and Nitsche, and Ritter and Rübsamen. A, the fertilized ovum. B, the resulting apterous and parthenogenetic Phylloxera. C, its eggs, from which, as the uppermost arrow indicates, there may arise similar apterous, parthenogenetic forms, or, as the horizontal arrow indicates, winged forms (D), which produce 'female' and 'male' ova (E1 and E2); from these the sexual generation arises, the female (F1) and the male (F2); the former lays the fertilized ovum (A).
As in all plant-lice, the advantage for the sake of which sexual reproduction was given up depends upon the fact that a practically unlimited food supply is at the disposal of these parasites of the vine, which can be made full use of during the proper season, and which, since every animal is female and produces eggs, results in an enormous increase in the number of individuals, and thus secures the continuance of the species. These insects emerge in spring from small fertilized eggs, which have lain dormant throughout the winter (Fig. 127, A), and they develop rapidly into wingless females (B), which, sucking the juice of the vine, multiply by producing large numbers of little white eggs (C). These develop without fertilization into similar wingless females. Several generations of females succeed each other, but then, usually from August onwards, differently formed winged females (D) make their appearance, and these, flying from plant to plant, effect the distribution of the species. But these, too, lay parthenogenetic eggs (E1 and E2), and from these there emerge, late in autumn, the members of the single bi-sexual generation, males and females (F1 and F2), both very minute and wingless, without a piercing proboscis, and thus incapable of taking food. These pair, and the female lays a single egg (A) under the bark of the vine, from which the leaves are now falling; this egg survives the winter, and from it in the following April or May there emerges once more a parthenogenetic female.
It could hardly be more plainly shown than it is by this case that the importance of amphimixis is something quite apart from reproduction and multiplication, for here the number of individuals is not only not increased by amphimixis, but is materially diminished, being indeed lessened by a half. By the retention of amphimixis, the species gains in this case no advantage except the mingling of two germ-plasms.
Something similar occurs in plants which exhibit alternation of generations, for instance the ferns, in which the sexual generation, the so-called prothallium or prothallus, contributes nothing to the multiplication of the plant, since only a single egg-cell is developed; and the same is true of the mosses. In both cases multiplication depends solely on the asexual generation, which, as the so-called 'moss-fruit' or 'fern-plant proper,' produces an enormous number of spores, in addition to multiplying by runners.
To sum up: we have seen that self-fertilization does occur in hermaphrodite animals, where otherwise the species would be in danger of extinction, but this is never the sole and exclusive mode of fertilization[25], for hermaphrodite species have always the possibility of securing inter-crossing of individuals, and that in various ways, whether by the intervention of 'primordial males' or by an occasional or a periodic alternation of self-fertilization and mutual fertilization. Pure parthenogenesis enduring through innumerable generations does appear to occur, but in most cases unisexual generations alternate with bi-sexual, so that a stereotyping of the germ-plasm with complete uniformity of ids is obviated.
[25] As to the cases Maupas has brought into notice, of permanent and apparently exclusive self-fertilization in Rhabditidæ (round worms), it seems fair to say that they have not been as yet sufficiently investigated to admit of a secure appreciation of their value in their theoretical bearings. Cf. Arch. Zool. Exper., 3rd ser., vol. viii, 1900.
We must now briefly consider the higher plants with reference to the maintenance of diversity in the germ-plasm through crossing.
We saw in an earlier lecture that most flowers are hermaphrodite, but that they do not fertilize themselves, and are adapted for crossing, since the pollen of one flower is carried by insects to the pistil of another, which cannot be reached by its own pollen, either because it ripens too early or too late, or because the stigma, notwithstanding its proximity, is so placed as to be out of reach of the pollen from the adjacent stamens. I showed, following the fundamental investigations of Sprengel, Charles Darwin, Hermann Müller, and other successors of Darwin, that the flowers may in a sense be regarded as the resultants of the insect-visits, since all their accessory adaptations—large coloured petals, fragrance, nectar, and even little minutiæ of colour and markings (honey-guides)—as well as their detailed shape, as seen in 'landing stages,' corolla tubes, and so on, are only intelligible when we refer their existence to natural selection. We assume that each of these adaptations secured some advantage for the species concerned, and that therefore their first beginnings as slight germinal varieties were accepted, and were brought gradually to their full expression by the united operation of germinal and personal selection. This at least is how we should express ourselves now that we have become acquainted with the factor of germinal selection. The advantage secured by every such improvement in a flower's means of attracting insects is obvious, as soon as it is established that cross-fertilization is more advantageous for the species than self-fertilization.
We have discussed this already; we saw that experiments instituted by Darwin proved that seedlings which had arisen through cross-fertilization were superior to those arising through self-fertilization, and that in many cases the mother-plant itself produced fewer seeds when self-fertilized than when cross-fertilized. This discovery afforded an explanation of the cross-fertilization of flowers by insects which Sprengel had previously observed. We understand how the flowers must have become so adapted through processes of selection that they were unable to fertilize themselves, but attracted insects, and, so to speak, compelled these to dust them with pollen from another plant of the same species. We also understand how self-fertilization remained possible for many flowers in the event of cross-fertilization through insects not being effected, since after a certain period of waiting, a curvature of the stamens or the pistil may take place and lead to the stigma being dusted with the pollen of the same flower. Obviously the development of fewer seeds is preferable to complete sterility. It is a well-known fact that peculiar inconspicuous and closed flowers, designed solely for self-fertilization, may occur along with the open flowers, as in the case of the so-called cleistogamous flowers of the violet (Viola) and the little dead-nettle (Lamium amplexicaule), and the phyletic origin of these becomes intelligible as soon as it is established that cross-fertilization is more advantageous than self-fertilization.
Now, however, it seems as if the fundamental proposition of this theory of flowers will have to be rejected. Not only do the cleistogamous flowers just mentioned exhibit a great fertility, not at all less than that of the open flowers of the same species which are adapted for cross-fertilization, but there is a small number of plants which produce seeds by self-fertilization alone. Thus in Myrmecodia cross-fertilization is absolutely prevented by the fact that the flowers never open, and according to Charles Darwin Ophrys apifera also reproduces by self-fertilization alone, and is nevertheless a thoroughly vigorous plant. There are several other cases of this sort, and particularly among the orchids, though the whole of the structure of their flowers is specially adapted for pollination by insects. Many of them are only rarely visited by insects, some not at all, we know not why, but it is readily intelligible that in such cases they should have adapted themselves to self-fertilization wherever that was possible. For this no great variation was necessary; it was enough that the pollinia, which formerly only became detached from their attachment at a touch or a push from an insect, should free themselves spontaneously. And this, according to Darwin, is what happens, for instance, in Ophrys scolopax, which at Cannes is frequently self-fertilizing. For the development of seed, however, it is not enough that the pollen should reach the stigma; the pollen-grain has to send out its tube and penetrate into the ovary, and in many orchids this does not happen; they are infertile with their own pollen. Various other plants are also non-fertile with their own pollen, for instance the common corydalis, Corydalis cava, or the meadow cuckoo-flower, Cardamine pratensis (Hildebrand).
How are we to reconcile these apparently absolutely contradictory facts? On the one hand, the innumerable devices for securing crossing lead us to conclude that it is necessary, or at least advantageous, and on the other we find a small number of plants which reproduce continually by self-fertilization and yet remain strong and vigorous. And again there are many plants which yield seed when fertilized with their own pollen, and others which remain absolutely sterile in the same circumstances, yielding no seed or very little, and there is indeed one on which its own pollen has the effect of a poison, for if it reaches the stigma the flower dies. If there is anything injurious in self-fertilization (Darwin), we can understand that it will be avoided, but how can it be continued so long in many cases, and even become in others the exclusive method of fertilization without visible evil results?
It seems to me that in these facts, established by observation, the results of two quite different processes have been confused, and that we can only gain clearness by studying them apart from one another; I mean the processes involved in the mechanism of fertilization and those involved in the mingling of the germ-plasms.
In many cases self-fertilization is said to yield less seed and weaker seedlings. Let us for the present take this statement as the basis of our consideration; it does not seem to me conceivable, though here I am not in agreement with views that have been expressed by others, that both effects should depend upon the same causes, for the smaller number of seeds cannot possibly depend upon the mingling of the two parental germ-plasms, and thus not upon the process of amphimixis itself, since the effect of the mingling does not make itself felt until the organism of the offspring is being built up. Of course the plant seed is the embryo of the young plant, but it will hardly be thought probable that its development could be absolutely prevented by the too close relationship of the two germ-cells, and thus the number of the developing seeds cannot depend on the quality of the ids co-operating in the segmentation-nucleus, but presumably on the number of ova awaiting fertilization in the ovary, which are reached by a pollen-tube and then by a paternal sex-nucleus. This again will depend upon the impelling and attracting forces of the pollen-grain on the one hand, and of the stigma and 'embryo-sac' of the flower on the other. In other words, the fertility of a flower with its own pollen will depend upon whether the two products of the flower are adapted for mutual co-operation, and in what degree they are so. We are here dealing not with the primary reactions of the germ-plasms, which are as they are and cannot be varied, but with secondary relations, which may be thus or thus—in short, with adaptations.
By what adaptations the pollen of a flower can be made ineffective for that flower is a question which we must leave the botanists to answer; in any case it must have been possible, and we see clearly that it depends upon adaptation when we consider the numerous stages which occur—from the rare case of the actually poisonous influence of self-pollination already noticed, to complete sterility, and from lessened fertility to greater or even perfect fertility. It is possible that chemical products, secretions of the stigma or the pollen-grain, or the so-called synergid-cells, have to do with this, or that the size and therewith the penetrating power of the pollen-cell in self-fertilization stand in inverse ratio to the length of the pistil, as has been proved in regard to heterostylism by Strasburger; but in any case it was possible for Nature, by means of slight variations in the characters of the male and female parts of the flower, to diminish the certainty of the meeting of the two germ-cells, even to the total exclusion of the possibility of any union of these.
If, then, self-fertilization had to be guarded against or at least rendered difficult because its consequences were injurious, all variations pointing in the direction of safeguarding would necessarily be preserved and increased. In many cases variations in the structure of the flower were sufficient; but when, as in Corydalis cava, the pollen could not readily be prevented from falling upon the stigma, the pollen might be made sterile as far as its own flower was concerned by a process of selection, in which on an average those plants would remain successful which produced the largest number of cross-fertilized seeds, and in this case those which did so were those whose pollen reacted most feebly to the stimulus of their own stigma.
Fig. 128. Heterostylism (Primula sinensis), after Noll. Two heterostylic flowers from different plants. L, the long-styled form. K, the short-styled form. G, style. S, anthers. P, p, pollen-grains. N, n, stigmatic papillæ of the long-styled and short-styled forms respectively. P, p, N, n, magnified 110 times.
That self-sterility in all these different degrees is not a primary character of the species, but an adaptation to the advantages of cross-fertilization, is apparent—if indeed it seems doubtful to any one—especially from cases of heterostylism. I refer to the dimorphism and trimorphism which Darwin discovered in many flowers, and which shows itself in the fact that flowers otherwise almost exactly alike, as, for instance, primroses, may exhibit a long style in some individuals, and in others a short one (Fig. 128). At the same time, there is a difference in the position of the stamens, which are placed higher up in flowers with short styles, and much lower down in those with long styles. Experiments have proved that the dusting of the stigma has the best results if pollen from the short-styled reaches the stigma of the long-styled form, or if pollen from the long-styled form reaches the stigma of the short-styled. Thus we have again to deal with an arrangement for crossing, an adaptation to the advantages of cross-fertilization, and we can in this case see the reason why the pollen has a different effect upon the two stigmas; the pollen-grains of the flowers with short style are larger than those of the flowers with long style, and as the length of the pollen-tube that can be sent out must depend upon the mass of protoplasm within the pollen-grain, it follows that the smaller pollen-grains will send out too short a tube to reach through the long style to the embryo-sac. In addition to this there is a difference in the papillæ of the stigmas, and it is possible that these may form an obstacle to the penetrating of pollen from a similar type. The process of selection which gives rise to such arrangements as we find in Primulas may easily be imagined, as soon as we are able to assume that cross-fertilization is more advantageous than self-fertilization as regards progeny, that is, as regards the continuance of the species.
We have already seen that uninterrupted self-fertilization is unknown among animals, but that it is not even very rare among plants, and this emphatically corroborates our previous conclusion, that the reason for which amphimixis was introduced as a normal event in nature is not to be sought for in the necessity for a renewing of life, or 'rejuvenation.' It cannot be a necessity, but only an advantage, which can in certain circumstances be dispensed with.
Although it is obvious enough that continued inbreeding in its most extreme form, self-fertilization, does not imply an absolute abandonment of amphimixis, the adherents of the rejuvenescence theory have regarded the unfavourable consequences of pure inbreeding as a confirmation of their assumption, according to which amphimixis is indispensable to the continuance of the life of the species, and it is therefore an important fact, if it can be proved, that continued self-fertilization can occur persistently, among plants at least, and yet not cause any injurious results to the species.
But how can this fact be understood from our point of view? How does it happen that crossing is striven after in so many different ways and yet so often given up again, and continued self-fertilization resorted to?
To this it may be answered, in the first place, that it is not, as far as we can see, for internal reasons that persistent self-fertilization becomes the rule; there is no peculiar condition of the germ-plasm which makes it disadvantageous or superfluous that the diversity of the id-combinations should be maintained; self-fertilization is due to external influences which bring it about that the plant has only the alternative of producing no seeds at all or of producing them by self-fertilization. In this connexion Darwin's experiments with orchids are particularly noteworthy.
In this very diversified order of plants there are numerous species whose flowers are infertile with their own pollen, although it does not reach the stigma in natural conditions, and therefore there was no necessity—as far as we can see—for guarding against self-fertilization by 'self-sterility.' These flowers are thus doubly adapted, so to speak, for crossing by means of insects. But as regards many of these, as well as many other modern orchids, insect-visits are very rare, and in some cases do not occur at all, and therefore these species cannot produce seed or can do so only exceptionally.
This is true of most of the Epidendra of South America, and of Coryanthus triloba of New Zealand, two hundred blossoms of which only yielded five seed-capsules, and also of our Ophrys muscifera and O. aranifera, the latter of which yielded only a single seed-capsule from 3,000 flowers gathered in Liguria. We might expect that the species in question must have become very rare, but this is not always the case, since each of these capsules contains an enormous number of seeds, sometimes many thousands. As soon as the visits of insects cease altogether, the species must die out in the particular locality concerned, unless it can revert to self-pollination and self-fertility. There is a whole series of species in which the stigma of the flower is sensitive to its own pollen, and in many of these an adaptation to self-fertilization has actually been effected, for the pollinia detach themselves from their anthers at maturity and fall upon the stigma. I have already mentioned Ophrys apifera, which, according to Charles Darwin, is no longer visited by insects, although its flowers still possess the structure required for insect-fertilization. This species has saved itself from extinction by the normal occurrence of self-fertilization.
This seems to me noteworthy in two respects. In the first place, it shows that pure self-fertilization need not necessarily result in a weakening of the species, and secondly, it affords a clear instance of a species being transformed in one minute character only, all the other characters remaining unaltered. In this case it was only the pollinia that required to vary a little in their mode of attachment and maturation, in order to effect the transformation of the flower for self-fertilization, and in point of fact that is all that has varied. The case is not relevant to our investigation at this moment, but cases of the kind can so rarely be clearly demonstrated that I cannot lose the opportunity of calling attention to it. The germ-plasm of this Ophrys must have varied at an earlier stage, for otherwise the detachment of the pollinia would not have become normal and hereditary, but it can only have varied to the extent that the structure of this one small part of the flower was affected by the variation; something must have varied in the germ-plasm that had no influence upon the other parts of the flower, that is, solely the determinants of the pollinia.
Let us return after this digression to our previous train of thought; we have to inquire how we can interpret the fact of continued self-fertilization without any visible injurious results to the species. If cross-fertilization be a material advantage as regards the continuance of the species, how can it be transformed into its opposite without evil effects? And there are no visible evil effects in Ophrys apifera. It is indeed not so abundant as Ophrys muscifera, or other allied species, but it certainly does not follow from that that it is on the way to extinction; certainly no decrease either of vigour of growth or of fertility can be observed.
If we inquire from the standpoint of our theory, how the composition of the germ-plasm must have altered through continual inbreeding, we have already found the answer—that through the reduction of the number of ids at the maturation of every germ-cell the diversity of the germ-plasm would gradually be lessened, that the number of different ids would thereby be lessened possibly even to the identity of the whole of the ids.
The consequences of such extreme uniformity of the germ-plasm would not, according to our theory, necessarily be that the species would be incapable of continued existence, but it would be that the species would become incapable of adaptations in many directions. Adaptations in one direction, such, for instance, as the variation in the mode of attachment and detachment of the pollinia of an Orchid, would still be possible. Thus a species which has long been perfectly adapted will be able to make the transition to inbreeding without injury to its chances of continued existence, if it be compelled by circumstances to do so. Species, on the other hand, which are still undergoing considerable transformations in many directions must be exposed by these to the danger of degeneration, just as happens in the artificial experiments with domesticated animals, whose secret weaknesses are greatly exaggerated by inbreeding.
We might be inclined to regard the effects of inbreeding as similar to those of parthenogenesis; they are certainly analogous, for both modes of reproduction must lead to a certain degree of uniformity in the germ-plasm. But there seems to me to be a difference and one which is not without importance.
In parthenogenesis no amphimixis occurs, but neither does any reduction of the number of the ids to one-half; all the ids present at the beginning of parthenogenesis are retained; they are only no longer mingled with strange ids. In inbreeding both amphimixis and reduction take place, but the former soon ceases to convey any really strange ids to the germ-plasm, but only the same as those which it already contains, so that a rapidly increasing monotony of the germ-plasm must result. To this must be added the possibility that among the few ids which now—many times repeated—form the germ-plasm, some must occur which exhibit unfavourable variational tendencies in one or many determinants, and then the same thing will occur which usually occurs in experimental inbreeding of domesticated animals, namely, degeneration of the progeny. In parthenogenesis the case is otherwise; unfavourable variational tendencies, as soon as they attain selection-value, are, so to speak, eliminated root and branch, because the individuals which exhibit them, and their whole lineage, are exterminated, without their having any effect upon the other collateral lines of descent. A purely parthenogenetic species will, therefore, not degenerate as long as individuals of normal constitution are present, for these reproduce with perfect purity. But if in later generations unfavourable variational tendencies crop up in the germ-plasm through germinal selection, the process of personal selection will be reinforced on these or on their descendants, and it is conceivable, and even probable, that in perfectly adapted species parthenogenesis may last for a very long time without doing any injury to the constitution of the species.
The same is true of purely asexual reproduction, to the investigation of which we shall now turn.
Let us leave out of account the simplest animals (Monera) without amphimixis, which we have already discussed. In simple animals reproduction by budding or by fission is frequent, or it occurs in alternation with sexual reproduction; in higher animals, Arthropods, Mollusca, and Vertebrates, asexual reproduction is wholly absent. In plants it plays an enormously greater part, and what is called 'vegetative reproduction,' which is purely asexual without any amphimixis, is to be found in all groups of plants, especially in the form of budding and spore-formation, besides which there is multiplication by runners, rhizomes, tubers, bulbs, and bulbils. In most cases there is, in addition to the purely asexual reproduction, so-called sexual reproduction associated with amphimixis, and often the sexual and asexual generations alternate with each other, so that 'alternation of generations' occurs, as is common in lower animals, especially polyps, medusæ, and worms.
But it sometimes happens among plants that the sexual reproduction is absent, and that a species reproduces by the asexual mode only, and this is the case which we must now consider more closely.
Let us first of all seek to gain clearness as to the composition of the germ-plasm in the case of purely asexual multiplication, and what conclusions may be drawn from this, and then let us compare these with the known observational data, and it will be apparent that in individuals which have arisen by budding the complete germ-plasm of the species must be contained; the number of ids will not only remain the same in the bud as it was in the mother plant, but the number of different ids will not be diminished. The case is analogous to that of pure parthenogenesis, in which the absence of the second maturation-division of the ovum allows the germ-plasm to retain the full complement of ids. Charles Darwin held that purely asexual multiplication was 'closely analogous to long-continued self-fertilization,' yet, as we have seen, according to our theory there must be a not inconsiderable difference between the two processes, depending on the fact that in exclusive self-fertilization the number of different ids is continually decreasing, while in purely asexual reproduction the germ-plasm loses nothing of the diversity of its ids. If, therefore, the germ-plasm in purely asexual reproduction no longer receives fresh ids through amphimixis, it at least loses none of those it formerly possessed. Although we cannot consider it adapted for entering upon new adaptations in many directions, yet we may expect that the species will continue to reproduce unchanged for longer than in the case of exclusive self-fertilization, the more so since all unfavourable variational tendencies which crop up are eliminated as soon as they attain to selection-value, and, as in the case of parthenogenesis, they are eliminated without being mingled with other lines of descent.
Let us take, for instance, the purely asexual reproduction which obtains in Algæ of the genus Laminaria, in regard to which it is stated that it multiplies only through asexual swarm-spores. There are quite a number of species of this large tangle, and if it should be established that in all these the spore-cells really do not conjugate, then the case would prove that the species of a genus can maintain a well-defined existence for a long time after amphimixis has been given up. But this would not be a proof of the possibility of species-formation, for that the ancestral forms of the Laminarians must have possessed amphigony may be assumed, since their nearest relatives exhibit it still. It cannot be proved, but there seems nothing against the assumption that these tangles have existed for a long time under uniform conditions, and have become adapted to these with a high degree of constancy.
The conditions are similar in the marine Algæ of the genus Caulerpa, the nearest relatives of which reproduce sexually, though they themselves, as far as is known, reproduce only by spores.
In the Lichens, which represent, as we have already seen, a life-partnership between Fungi and Algæ, amphimixis appears not to occur at all; the unicellular Alga reproduces by cell-division, the Fungus by producing a great number of swarm-spores, which do not conjugate with one another. As far as the Alga is concerned we might perhaps suppose that the simplicity of its structure makes it possible for it to dispense with a constant recombination of its few characters to bring about the most favourable composition in its idioplasm; in support of this we may note that even the life-long combination with the Fungus has caused no visible variation in the Alga, as we must conclude from the fact that these Algæ can also live independently, and that the same species of Alga may combine with several different Fungi to form different species of lichen, just as the same Fungus may also form part of several species of lichen. We might also imagine that we have here no more than a direct influence of the Alga and Fungus upon one another, and that there is no adaptation to the new conditions of life at all, yet that can hardly be seriously maintained in regard to species which live under such definite and diverse conditions. It now seems to be established—contrary to the older statements—that the lichen-fungus only reproduces asexually, and in face of this it seems to me that nothing remains except to make the assumption that lichens formerly possessed sexual reproduction, but that they have lost it, though whether all have done so is, perhaps, not yet quite certain.
The same assumption must be made in regard to the Basidiomycetes among the Fungi, and for most of the Ascomycetes, for in these groups of Fungi sexual reproduction has only been demonstrated 'with certainty in a few genera.' That in these cases also there has been a degeneration of amphigony, until it has completely disappeared, seems probable from the two other groups of Fungi, the Zygomycetes and Oomycetes, since in these 'a reduction of sexuality amounting in some cases to complete disappearance' can be demonstrated even in existing forms. But whether it may be assumed that the Fungi which are now asexual are no longer capable of new adaptations, and whether their parasitic habit may be regarded as making up in some way for the lack of the remingling of the germ-plasm, as the botanist Möbius supposes, I am not able to decide. It is obvious that data in regard to amphimixis among the Fungi are still incomplete, and recent investigations lead us to suspect that sexual mingling may not be absent, but only disguised. Dangeard, Harold Wager, and others have observed that a fusion of nuclei precedes the formation of spores, and this may be regarded as amphimixis, although the conjugating nuclei belong to cells of the same plant and sometimes even to the same cell. But although we are here dealing with a set of facts which cannot yet be satisfactorily formulated in terms of our theory, it is nevertheless not contradictory to it that amphimixis should be wholly absent in the higher Fungi. But the fact would be contradictory to the unadulterated rejuvenescence-theory, for if amphimixis were really a condition of the continuance of life, no species—as we have already said—could continue to exist without it for countless generations.
Fig. 38 (repeated). A fragment of a Lichen (Ephebe kerneri), magnified 450 times. a, the green alga-cells. P, the fungoid filaments. After Kerner.
The same argument holds true for the higher plants, which have become purely asexual under the influence of cultivation. I refer to many of the well-marked varieties of our cultivated plants which multiply exclusively, or almost exclusively, by means of tubers and slips, as is the case with the potato, the manioc, the sugar-cane, the arrowroot-plant (Maranta arundinacea), and others. All these facts can easily be reconciled with our interpretation of the meaning of amphimixis, although the attempt to range them as evidence against our theory has more than once been made. We have thus arrived at the conclusion that while many-sided adaptations, that is, variations which transform the plant in accordance with the indirect influences of new conditions of life, cannot be brought about without a persistent mingling of germ-plasms, simple modifications may readily appear although amphimixis is altogether absent. If a wild plant be permanently transferred to a well-manured culture-bed, it is probable that certain changes will occur in it, either gradually or at once. But these are not adaptations; they are, so to speak, direct reactions of the organism which do not even require selection to make them increase, but depend upon the influencing of certain determinants of the germ-plasm, and which, like all germinal variations, will follow their course steadily until a halt is called either by germinal or by personal selection. When a given plant is exposed to these new and artificial conditions, the changes in question make their appearance sooner or later, and follow their course, and go on increasing as long as that is compatible with the harmony of the structure and functioning of the plant, this depending, as in all individual development, on the struggle between the parts, that is to say, on histonal selection. Only in this respect is the utility or injuriousness of the change of importance, for personal selection, the struggle between individuals, does not affect plants which are under cultivation.
That such modifications may increase and may persist through many generations, even with asexual multiplication, depends upon the fact that the budding cells contain germ-plasm, as well as the germ-cells, and if particular determinants of the germ-plasm in general are caused to vary by these new influences, the variation may be transmitted from bud to bud, from shoot to shoot, and so go on increasing as long as the new conditions persist, as well as in amphigonic (bisexual) reproduction, where they are transmitted from germ-cell to germ-cell. It is not inconceivable that an individual adaptation, that is to say a useful adjustment, might be effected in the course of asexual reproduction, although it is improbable that direct influences would give rise to just those changes which would be useful under the new conditions. But there are a number of cases which have been interpreted in this way. In several of the cultivated plants named, the reproductive organs have themselves degenerated, either only the male, or only the female, or both at the same time; and some observers, accepting the hypothesis of an inheritance of functional modifications, have regarded this as the direct result of disuse during the long period of asexual reproduction.
Leaving out of account this erroneous presupposition, we may ask how asexual reproduction, such as that of the potato by tubers instead of by seed, which has gone on exclusively for several centuries, could exercise any influence upon the flowers and seed-forming of this species? In point of fact it has exercised none in most potatoes, for the flowers and seeds are just as fertile now as they were when the potato was first discovered.
Whether the pollen of a flower is utilized in one or other of its thousands of pollen-grains by reaching the stigma of another plant of the same species, or whether all the pollen-grains are uselessly scattered abroad, cannot possibly affect the flower so as to cause degeneration; the theory of disuse cannot be applied in this case. What is true of the potato holds good also of the manioc (Manihot utilissima), but, on the other hand, many of the best varieties of common fruits—pears, figs, grapes, pine-apples, and bananas—are seedless. In Maranta arundinacea 'the whole wonderful structure of the flower has persisted, but the pollen-grains, that is the germ-cells, are wanting.' Whether this implies a permanent degeneration of the sexual organs, that is to say, one that is embodied in the primary constituents of the species, or whether it is only the result of over-abundant nourishment, or of other causes in the circumstances affecting the particular plant, can only be decided by experiment. Probably both occur. The common ivy, for instance, does not now blossom in the northern parts of Sweden and Russia, but it does so still in the southern provinces. If plants were brought to us from the most northerly zone of distribution, they would in all probability flower and bear fruit with us, and in that case the absence of bloom in these plants must have been a direct effect of the cold climate. But it is quite conceivable that cultivated plants have in many cases become hereditarily infertile, when they are constantly propagated only by means of buds, layering, and so on, not however because of any direct effect of this mode of propagation, but through chance germinal variations. For in regard to many of them man has lost all interest in the flowers and fruit, as, for instance, in the case of the potato; in other cases he is even interested in procuring seedless fruits.
In the first case he will quite readily make use of plants with imperfect flowers for propagating, if they are otherwise fit and exhibit what he wants in other respects; in the second case, he will give a preference to individuals with seedless fruits, and thus increase and strengthen the tendency to degeneration of the seeds in the race concerned.
All these cases are quite in harmony with our conception of amphimixis, which, now that we have investigated the facts throughout the animate kingdom, we may sum up in the following propositions. In the whole organic world, from unicellular organisms up to the highest plants and animals, amphimixis now means an augmentation of the organism's power of adaptation to the conditions of its life, since it is only through amphimixis that simultaneous harmonious adaptation of many parts becomes possible. It effects this by the mingling and constant recombination of the germ-plasm ids of different individuals, and thus gives the selection-processes the chance of favouring advantageous variational tendencies and eliminating those which are unfavourable, as well as of collecting and combining all the variations which are necessary for the further evolution of the species. This indirect influence of amphimixis on the capacity of organisms for surviving and being transformed is the fundamental reason for its general introduction and for its persistence through the whole known realm of organisms from unicellulars upwards.
The reason for its first introduction among the lower forms of life must have been a direct effect which had a favourable influence on the metabolism, and this is so far coincident with the subsequent import of amphimixis, inasmuch as it may be regarded not only as a heightening of the power of adaptation, but as an immediate and direct increase and extension of the power of assimilation. In any case, amphimixis is not necessary to the actual preservation of life itself, but it does bring about a wealth and diversity of organic architecture which without it would have been unattainable.
If amphimixis has been abandoned in the course of phylogeny by isolated groups of organisms, this has happened because other advantages accrued to them in consequence, which gave them greater security in the struggle for existence; but it must be admitted that they thereby lost their perfect power of adaptation, and that they have thus bartered their future for the temporary securing of their existence.
In addition to this variational influence, amphimixis has also played a part in the evolution of sharply defined organic types, especially of specific types; but of this we shall have more to say later on.