1902 Encyclopedia > Biology > The Alteration of Generations. Individuality.

Biology
(Part 18)




The Alteration of Generations. Individuality.

In many plants and animals which multiply both a sexually and sexually, there is no definite relation between the agamogenetic and the gamogenbetic phenomena. The organism may multiply asexually before, or after, or concurrently with, the occurrence of sexual generation.

But in a great many of the lower organisms, both animal and vegetable, the organism (A) which results from the impregnated germ produces offspring only agamogenetically. It thus gives rise to a series of independent organisms (B, B, B,…), which are more or less different from A, and which sooner or later acquire generative organs. From their impregnated germs A is reproduced. The process thus described is what has been termed the "alternation of generation" under its simplest form, form example, as it is exhibited by the Salpae. In more complicated cases, the independent organisms which correspond with B may give rise agamogenetically to other (B1), and these to others (B2), and so on (e.g., Aphis). But however long the series, a final term appears which develops sexual organs, and reproduces A. The "alteration of generation" is, therefore, in strictness, an alteration of asexual with sexual generation, in which the products of the one process differ from those of the other.

The Hydrozod offer a complete series of gradations between those cases in which the term B is represented by a free, self-nourishing organism (e.g., Cyanaea), through in which it is free but unable to feed itself (Calcophorudae), to those in which the sexual elements are develop in bodies which resemble free zooids, but are never detached, and are mere generative organs of the body on which they are developed (Cordylophora).

In the last case, the "individual" is the total product of the development of the impregnated embryo, all the parts of which remain in material continuity with one another. The multiplication of mouths and stomachs in a Cordylophora no more makes it an aggregation of different individuals than the multiplication of segments and legs in a centipede converts that Arthropod into a compound animal. The Cordylophora is a differentiation of a whole to many parts, and the use of any terminology which implies that it results from the coalescence of many parts into a whole is to be deprecated.

In Cordylophora the generative organs are incapable of maintaining a separate existence; but in nearly allied Hydrozoa the unquestionable homologues of these organs become free zooids, in many cases capable of feeding and growing, and developing the sexual elements only after they have undergone considerable changes of form. Morphologically, the swarm of Medusae thus set free from a Hydrozoon are as much organs of the latter, as the multitudinous pinnules of a Comatula, with their genital glands, are organs, of the Echinoderm. Morpholigically, therefore, the equivalent of the individual Comatula is the Hydrozoic stock + all the Medusae which proceed from it.

No doubt it sounds paradoxical to speak of a million of Aphides, for example, as parts of one morphological individual; but beyond the momentary shock of the paradox no harm is done. On the other hand, if the asexual Aphides are held to be individuals, it follows, as a logical consequence not only that all the polypes on a Cordylophora tree are "feeding individuals," and all the genital sacs "generative individuals," while the stem must be a "stump individual," but that the eyes and legs of a lobster are "occural" and "locomotive individuals". And this conception of the origin of the complexity of animal structure which is wholly inconsistent with fact.





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