The evolution of sex / by Professor Patrick Geddes and J. Arthur Thomson.

  • Patrick Geddes
Date:
[1900]
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The evolution of sex / by Professor Patrick Geddes and J. Arthur Thomson. Source: Wellcome Collection.

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    2IO THE EVOLUTION OF SEX. heads occurs. The tapeworm, on this view, is an adult sexual bladder- worm, and the joints are only highly individualised segments. Of the parthenogenetic cycles in crustaceans and insects, the juvenile reproduction of some of the latter, and the true alternation of generations in some tunicates, enough has already been said. Von Jhering is responsible for starting the paradox, that in higher animals a mother may bring forth her grandchildren. He refers to the case of the hycena-like carnivore Praopus, where a single ovum gives rise to eight embryos, which are thus in a pedantic sense grandchildren ! The frequent occurrence of twins in all groups, the remarkable case of an earth¬ worm {Lumhriciis trapezoides) in which a double embryo is constant, and the morphological resemblance of polar globules to abortive germs, led Von Jhering to maintain that the origin of multiple embryos from a single ovum is the primitive and normal condition, and that the development of only one is secondary and adaptive. The data are hardly sufficient for such a striking conclusion. § 9. Occurrence of Alternations in Plants.—In the lower plants, algae and fungi, an alternation between spore-produeing and truly sexual generations is frequent. In mosses and ferns it is almost constant, and yet more marked. Occasionally either spore-formation or sex-cell formation may be suppressed, and the life-history thus sim])lified. In a few of the higher plants both are exceptionally suppressed, and we have thus a reversion to a purely vegetative process, just as if a hydra went on giving off daughter-buds without ever becoming sexual. In the flowering plants, what corresponds to the sexual generation of a fern is much reduced ; it has come to remain continuous with the vegetative asexual generation, on which it has reacted in subtle physiological influence. Just as in the higher animals, alternation of generations flnds at most only a rudimentary expression. Ì5 10. Heredity in Alteriiating Generations.—The problem of the relative constancy of inheritance is now in part solved by the theory of germinal continuity. '1Ъе ovum which develops into an offspring is virtually continuous, either in itself or through its nucleus, with the ovum which gave rise to the parent. A chain of ovum-like cells is only demonstrable in a few cases; but Weismann overcomes this difficulty, by supposing that what really keeps up the protoplasmic tradition or con¬ tinuity between the parental ovum and the next generation, is a specific and stable portion of the nucleus,—the germ-plasma. When a medusoid goes off from a hydroid, it carries with it a legacy of this germ-plasma, continuous with that which gave rise to the hydroid. This legacy forms the reproductive elements of the medusoid, which in turn give rise to hydroids.