Page:Popular Science Monthly Volume 16.djvu/184

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or hermaphroditism; 2, that bisexual reproduction was developed out of non-sexual reproduction; and, 3, that non-sexual reproduction is but an unessential modification of the ordinary process of growth.

4. Facts which furnish a Key to the Process of Derivation.—There are certain facts which throw light on each of these steps, but, as might be expected, the light is far clearer on the higher steps, because these were also the last taken.

(a.) Facts which bear on the Last Step, viz., the Derivation of Unisexuality from Bisexuality.—These facts are taken from both the vegetable and the animal kingdom, but especially the former. They are comprehended under the general term "cross-fertilization of bisexuals."

Plants.—It is a familiar fact that most plants are bisexual, i. e., have both ovary and spermary (anther-cell), in the same individual plant and in the same flower; and that nearly all such cases are capable of self-fertilization. But Mr. Darwin has shown that, although capable of self-fertilization, yet cross-fertilization—i. e., the fertilization of the ovules of one flower, or, still better, of the flowers of one plant by the pollen of another—produces more seeds, larger seeds, and stronger seedlings; in other words, produces better results. Now, it is a law which necessarily results from the principle of the survival of the fittest that Nature ever strives to secure better results. Therefore, she immediately sets to work to contrive methods of insuring cross-fertilization and preventing self-fertilization. The cross-fertilization is insured—1, by winds, aided by the lightness of the pollen; and, 2, by insects which carry the pollen from flower to flower. The beauty, the fragrance, and the honey of flowers are undoubtedly intended primarily to attract insects, and thus to insure cross-fertilization. But this alone is not sufficient. It is necessary also to prevent self-fertilization. This is done sometimes, as in orchids, by sticking together the pollen in masses by means of a gummy substance, so that it can not fly, and placing these masses entirely beyond the reach of the stigma, and sometimes by the maturation of the ovules and of the pollen at entirely different periods. In these cases the plant is wholly dependent upon insects for their fertilization, and we accordingly often find the most curious and ingenious contrivances in the structure of the flower to make sure that there be no failure in this respect. In other cases self-fertilization is still more effectually prevented by a separation of the sexes in different flowers (Monœcia), or in different plant individuals (Diœcia)—of course, winds and insects being still the carriers between the two sexes. This separation of the sexes was undoubtedly a gradual process. In bisexual plants, habitually cross-fertilized by winds or by insects, the one organ or the other became aborted until first only rudiments remained, and finally even these are lost and unisexuality is complete. These stages are sometimes detectable.