PART II THE CAUSES OF THE PERSISTENCE OF ORGANS CHAPTER I SURVIVAL OF ORGANS WE have shown how and why organs may become rudimentary and tend to disappear. In many cases the disappearance is complete; and the organ may not even reappear temporarily in the course of the individual development. This disappearance is, however, by no means universal. Even apart from the phenomena of recapitulation, rudimentary organs may persist in the adult, and sometimes, even although organs have ceased to be functional, they persist without degenerating. We have now to consider why in such cases degenerative evolution does not result in complete obliteration of such organs. § 1. Unfunctional organs that are not rudimentary. ABSENCE OF VARIATIONS.- -There are some plants such as Ficaria1 ranunculoïdes and Lysimachia 1 Lysimachia Nummularia occasionally produce seeds in some valleys of the Pyrenees, and Errera has shown us specimens grown from seeds coming from the shores of the lake of Quatre-Cantons. 292 Nummularia, the flowers of which hardly ever produce seeds. How is it that in such species flowers are still produced? The probable explanation of this anomaly is, that for the disappearance of flowers there would have to be produced individuals with this advantageous variation. It is the case, however, that the Ficaria and the Lysimachia reproduce most actively by asexual methods, and variations are extremely rare in cases of these modes of reproduction. The result is that these species having begun to form sterile flowers continue to produce them through simple lack of variation. An analogous case is presented by Elodea Canadensis. This unisexual plant is represented in Europe by only female plants. These plants have multiplied asexually so luxuriantly that in Holland they began to choke up the canals, and it became necessary to make provision in the budget of that country for the extermination of the pest. The plants are, of course, able to multiply only asexually, as the female flowers cannot be fertilized, and these useless flowers have been maintained simply from the absence of variations. Stratiotes aloïdes, a plant belonging to to the same family as Elodea, is practically only represented by male individuals. Females are extremely rare, and none the less the male flowers are pro duced, although in the vast majority of cases they must be useless. W. Burck has called attention to, without endeavouring to explain, other instances of the persistence of functionless organs. A large number of Anonaceæ bear flowers which do not open, and which are self-fertilizing (cleistogamous flowers). None the less, they have retained the corolla, the original purpose of which was to attract insects.1 Burck has called our attention to the circumstance that several species belonging to the same genus produce cleistogamous flowers, and that it is improbable that this condition has been acquired independently by these species. One would thus have to admit that the original type must be very remote, as it has given rise to descendants of specific distinction, and yet the useless corolla has persisted through the long series. Parallel examples may be found among animals. Machaerites is an insect which inhabits caves in North America. The females are quite blind; the males, on the other hand, have preserved, or seem to have preserved, well-developed eyes; but are these eyes real? An abyssal fish, Ipnops,2 seems as if it had enormous eyes extending from the corner of the snout some distance along the neck, but these organs are not really eyes; they 1 W. Burck, Ueber Kleistogamie im weiteren Sinne und das Knight-Darwinsche Gesetz (Ann. Jard. Bot. Buitenzorg, viii., p. 122, 1889). 2 Dollo, La Vie au sein des mers, p. 242, Paris, Baillière, 1891. are light-producing organs, and the fish are in reality blind. This may be the case also in the males of Machaerites. It may also be the case that the male has an opportunity for using eyes absent in the case of the female, the males sometimes leaving the caves, the females remaining within them. Something analogous to this occurs in the case of eels: the males remain always in the sea while the females rejoin them only for purposes of reproduction. Moreover, there is still a third hypothesis, that the male of Machaerites became an inhabitant of caves later than the female, and has not yet had time for the loss of its eyes. § 2. Unfunctional organs which persist as rudiments. It is outside our purpose to discuss here the numerous cases of organs reduced through adaptation, such as the leaves reduced to serve as protectors of young buds (Phyllocactus, fig. 78), or the wings of the ostrich which, although much reduced, are supposed to assist the bird in running. The utility of such organs explains their persistence; we are concerned here with organs which, although useless, persist in a reduced form. 1. ABSENCE OF VARIATIONS.-In discussing organs which, although without function, have persisted in a complete state, we attributed the persistence to absence of variations. It is probable that the same cause operates in maintaining useless vestiges. It is to be noticed, however, that the variability of vestiges is frequently considerable. The flowers of Asparagus officinalis are sometimes, although rarely, hermaphrodite. Usually they are unisexual and exhibit the organs of the other sex in every conceivable stage of degeneration. It is probable that the unisexual condition has been acquired recently, and that there has not yet been time for the operation of natural selection to cause the disappearance of the useless organs.1 1 We have actual knowledge of the mode of disappearance of an organ in one case, and can see the part played by variability. In Phyllanthus speciosus (Xylophylla arbuscula) the adult plant has three kinds of branches: vertical branches, with rudimentary leaves; oblique branches which spring from the axils of these leaves and themselves bear in two rows very rudimentary leaves; and flat branches which are the chief organs of assimilation of the plant and which also bear rudimentary leaves in two rows. In the seedling, on the other hand, there are formed after the two cotyledons, one or two completely developed assimilating leaves upon an upright stem (fig. 84, A). The flat branches grow from the axils of these, and bear, unlike the flat branches of the adult, assimilating leaves; higher up the vertical stem bears only rudimentary leaves with flat branches in their axils. It may happen, however, that the seedlings bear, directly after their cotyledons, rudimentary leaves (fig. 84, B) and in this case the reduction of the leaves is present not only in the leaves borne upon the vertical stem but in those borne on the flat branches. These latter bear only a small number of assimilating leaves. Thus, we have in this plant a remarkable example of incomplete recapitulation: the seedling preserves in a functional condition organs that are rudimentary in the adult, but the species furnishes instances where these leaves cease to be functional even in the seedling. |