Page:Popular Science Monthly Volume 43.djvu/497

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cation of somatic cells, descending from a single ovum, may go; because it will be contended,-with some reason, that each of the sexless Aphides, viviparously produced, arose by fission of a cell which had descended from the original reproductive cell. I cite it merely to show that when the cell-products of a fertilized ovum are perpetually divided and subdivided into small groups distributed over an unlimited nutritive area, so that they can get materials for growth at no cost, and expend nothing appreciable in motion or maintenance of temperature, cell-production may go on without limit. For the agamic multiplication of Aphides has been shown to continue for four years, and to all appearance would be ceaseless were the temperature and supply of food continued without break. But now let us pass to analogous illustrations of cause and consequence open to no criticism of the kind just indicated. They are furnished by various kinds of Entozoa, of which take the Trematoda investing mollusks and fishes. Of one of them we read: "Gyrodadylus multiplies agamically by the development of a young Trematoda within the body, as a sort of internal bud. A second generation appears within the first, and even a third within the second, before the young Gyrodactylus is born."[1] And the drawings of Steenstrup, in his Alternation of Generations, show us, among creatures of this group, a sexless individual, the whole interior of which is transformed into smaller sexless individuals, which severally, before or after their emergence, undergo similar transformations—a multiplication of somatic cells without any sign of reproductive cells. Under what circumstances do such modes of agamic multiplication, variously modified among parasites, occur? They occur where there is no expenditure whatever in motion or maintenance of temperature, and where nutriment surrounds the body on all sides. Other instances are furnished by groups in which, though the nutrition is not abundant, the cost of living is almost unappreciable. Among the Cœlenterata there are the Hydroid Polyps, simple and compound; and among the Mollusca we have various types of Ascidians, fixed and floating, Botryllidæ and Salpæ.

But now from these low animals, in which sexless reproduction, and continued multiplication of somatic cells, is common, and one class of which is named "zoöphytes," because its form of life simulates that of plants, let us pass to plants themselves. In these there is no expenditure in effort, there is no expenditure in maintaining temperature, and the food, some of it supplied by the earth, is the rest of it supplied by a medium which everywhere bathes the outer surface: the utilization of its contained material being effected gratis by the sun's rays. Just as was to be ex-

  1. A Manual of the Anatomy of Invertebrated Animals, by T. H. Huxley, p. 206.