continually circumnutating, though often on a small scale. Even the stems of seedlings before they have broken through the ground, as well as the buried radicles, circumnutate as far as the pressure of the surrounding earth permits. In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements. Thus the great sweeps made by the stems of twining plants and by the tendrils of other climbers result from a mere increase in the amplitude of the ordinary movements of circumnutation. The position which young leaves and other organs ultimately assume is acquired by the circumnutating movement being increased in some one direction. The leaves of various plants are said to sleep at night, and it will be seen that their blades then assume a vertical position through modified circumnutation, in order to protect their upper surfaces from being chilled through radiation. The movements of various organs to the light, which are so general throughout the vegetable kingdom, and occasionally from the light, or transversely with respect to it, are all modified forms of circumnutation; as, again, are the equally prevalent movements of stems, etc. towards the zenith, and of roots towards the

centre of the earth.

In accordance with these

conclusions, a considerable difficulty in the way of evolution is in part removed, for it might have been asked, How did all their diversified movements for the most different purposes first arise? As the case stands, we know that there is always movement in progress, and its amplitude or direction, or both, have only to be modified for the good of the plant in relation with internal or external stimuli.” 1

Thus the great work of observation and reasoning began with an effort to explain the power of climbing among plants under the theories of descent and natural selection; passed on to the prediction of the universal movement of circumnutation and its verification; and closed by explaining all the other highly specialized and remarkable movements of plants and plant organs as modifications of the same general but unapparent movement. The principal difficulty

at first was the fact that climbers were found throughout the plant kingdom, and could not have been descended from a common climbing ancestor. By the investigations of Darwin and his son, not only were the different methods of climbing shown to be modifications of the twining movements of the stem, but it and all the 1 Power of Movement in Plants, p. 4.

other movements of plants were shown to be modifications of a universal movement. What was at first a difficulty in the way of evolution became, like the structure of the flowers of orchids, the ocelli of the peacock,, and the expression of the emotions, one of the strongest supports of the theory.

12

EVE

XII.

UNVERIFIED DEDUCTIONS.

VERY apparently insignificant fact was full of meaning to Darwin; and he made it the occasion for what he used to call "fool's experiments." His speculative powers employed themselves as actively and energetically on the details of his investigations as on their larger outlines; but he was as ruthless in testing and rejecting his speculations as he was facile in making them. When, however, he had once established a principle, he followed out the deductions from it with as much confidence as if he had already secretly seen the facts whose existence he suspected or thought probable. It is important to note the caution with which he usually stated his anticipations, and to contrast with it the energy and confidence with which he sought and worked out the facts. He spent his life establishing the consequences of his theories, but with all his fidelity and persistence he had to leave many things unproved; some for lack of time, others

He

because of the inaccessibility of the facts. had the satisfaction of living to see the whole biological world applying itself to the work of bringing out the consequences of his theories. It is not a part of the present purpose, even if it were possible, to follow out the logical history of the subsequent work based on those theories. But it will be of interest to notice a few instances in which he made deductions which he could not verify. Some of these have been since verified by others, and some still remain unverified. They vary all the way from confident predictions to vague expressions of a wish that some one would make observations that he thought would bear fruit.

There is one instance of a difficulty in the way of Darwin's theories which is of especial importance. He outlined a possible explanation, but the difficulty has proved itself so stubborn that even some of his adherents feel that his theories could not face many of the same kind. The problem is almost exactly similar to that of climbing plants, and its interest is increased by some recent work that has been done towards its solution.

Electric organs occur in various unrelated species of fishes, and differ so widely in their position in the body, their mode of innervation,