Page:Popular Science Monthly Volume 16.djvu/189

 transitional stages still in existence, almost certain, that in other cases sexuality was reached by a slower process and at a later period. It is this slower process which I now wish to trace in outline:

(a.) Fission.—In the lowest animals the individual cells which form their structure are almost wholly independent. The independent life of the cell is strong, the common life of the aggregate is feeble. By continued cell-multiplication, the aggregate, becoming too large to be held together by the common life, divides. Thus arises the lowest form of reproduction, viz., by fission. Many lower animals still practice this mode.

(b.) Budding on Any Part.—In the next step excess of growth occurs on any part indifferently, gives rise to a tubercle which grows into a bud, assumes the structure of the parent stem, and finally separates to become a new individual. This is higher than the last, because the original individual is not sacrificed, but only a part separated. Many larval medusæ and many polyps still practice this mode.

(c.) Budding on a Special Part.—In the last case the budding occurs in any part. In the next step a particular part is selected, and to it is assigned the function of forming buds which form new beings. Many larvæ of medusæ belong to this category; for they bud only on the mouth-disk. This is a higher form than the last, inasmuch as the assignment of a function to a particular place, or localization of a function, is an invariable step in evolution, and always attended with better results.

(d.) Special Budding Organ, internal.—The next step was probably the relegation of the function of producing buds to an internal organ, as being far safer and more certain of success, which organ thus becomes by position and function strongly analogous to an ovary. This is the case in larval aphids. The reproductive organ of these larvæ has been regarded by some as an ovary, by others as an internal budding organ. It is certainly not a true ovary, but rather perhaps an organ uniting the yet undifferentiated functions of ovary and spermary, an organ producing cells having the properties of both germ-cells and sperm-cells, and therefore capable of directly forming an embryo by cell-multiplication.

(e.) Differentiation of Sexual Elements.—The interior reproductive organ last described next forms two kinds of cells which by conjugation produce the ovum; the sexual elements are now differentiated, but not yet the sexual organs. It is not absolutely certain that this condition actually exists in any species now living; but it is probable that it does. According to Kleinenberg, the reproductive organ of the hydra produces both ovules and spermatozoids. In many mollusks and polyps the separation of the ovary and spermary is not yet made out. In some gasteropods the epithelial cells of the oviduct seem to become mother-cells, in which are produced spermatozoids. The