Page:Popular Science Monthly Volume 64.djvu/457

Rh higher animals plants are but loose and unspecialized aggregations of cells, and yet among them also sexual differentiation has made great progress, and in some orders contrivances to insure cross-fertilization are highly developed.

The extent to which conjugation exists among the lower groups is not yet determined. That it may be omitted for many generations of a simple organism should not be taken to mean that it is entirely absent or has no importance, since among the higher animals, where cross-fertilization is recognized as indispensable, the growth of the body to maturity requires millions of cell-divisions, each of which would mean a new generation in a unicellular species. The supposed absence of sexual reproduction in certain parasitic and saprophytic groups is a confirmatory exception, in view of the obvious degeneration of such organisms.

To the many speculations on the purpose of sex and cross-fertilization it can do no harm to add the conjecture that the presence of moderately diverse qualities of protoplasm facilitates cell-division. Some have held that the function of sex is to assist evolution by producing variations, and others that it neutralizes variation by maintaining a stable average. From the kinetic point of view it appears that symbasis, as represented by the phenomena of sex and of cross-fertilization, is not an impediment to evolution, nor a device to cause variation, but a means of communicating it. Variations appear without sex, and may even be accumulated, as by the adding of one bud variation to another in plants propagated by grafts or by cuttings, like the breadfruit, apple and banana. Such progress is, however, slow and halting, and is accompanied by a decline in reproductive fertility. Symbasis not only sustains the vitality of organisms already evolved, but it is directly responsible for the upbuilding of the complex structure and vital economy of the higher plants and animals, and it builds the faster when by the differentiation of sexes two sets of variations can be accumulated.

To symbasis is due also the arrangement of organisms in the coherent groups called species, or what may be termed the specific constitution of life. Conjugation is the means of symbasis, as division is of reproduction. Sexual and other dimorphism, and the numerous specializations, devices and instincts by which cross-fertilization is secured, are aids to symbasis, just as the spore-sacs, ovaries and placentæ facilitate reproduction. The phenomena of reproduction and those of sym-