Page:Popular Science Monthly Volume 45.djvu/515

Rh resemblance between the moth and the wasp; any moth in which that resemblance was in any degree unusually marked had therein an advantage, and tended to be in some measure left alone by its enemies; in thus escaping it could transmit its peculiarities of form and hue to its progeny, and so on, until in the rapid succession of insect generations, amid the equally rapid destruction of comparatively unprotected moths, the present striking similarity was at last attained. The study of mimicry of this type has from an unexpected quarter afforded singular confirmation of the theory of natural selection; in many cases the evidence of transformation within comparatively recent time is distinct—in the bee hawk moth, for example, the wings as they emerge from the chrysalis are thinly clothed with scales of ancestral derivation which are shaken off in the insect's first flight, with the result that the bumblebee is the better and more gainfully resembled.

As in the study of insects, so in that of plants—observation in the field at every stage of growth and development is needful to supplement the disclosures of the microscope and the dissecting needle. Many species, of which the milkweed blossom may stand as a type, are absolutely dependent on insects for their fertilization. How, therefore, can they be fully known in the laboratory and the herbarium? There is no more remarkable adaptation in Nature than that by which an orchid and the insect which continues its race conform to the outlines of each other. And hundreds of flowers less conspicuous than this orchid present perfumes, colors, and mechanism for attracting, seizing, and even imprisoning their insect visitors, which might well be the work of deliberate contrivance instead of inevitable selection from varying scents, hues, and forms of those which prove slightly more serviceable than others. That clover, peas, and other legumes receive their nitrogen from the air has long been suspected by agricultural chemists. The details of the process disclose one of the most curious interdependencies in the realm of Nature. Prof. Hellriegel has discovered nodules on the rootlets of the plants, tenanted by parasitic bacteria, which, while consuming a little of the substance of the plant, pay a handsome rent in the compounds of nitrogen which they build out of the air and pass to the fibers that harbor them. These microscopic purveyors, when bred and sown of set purpose, yield vastly increased crops of clover, alfalfa, peas, cow peas, beans, and lupine. Of this abundant testimony was presented at the Columbian Exhibition in the display of the Experiment Stations in the Agricultural Building.

Here, indeed, we come to the distinctive standpoint whence knowledge sweeps its new horizons: its outlook upon Nature as a whole, as a system intelligible only in the mutual interaction of