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But other large families, normally composed of non-climbers, contain one or a few climbing species, as, for example, the hop in the nettle family. The power to climb is so striking a character, and is so plainly useful to the plant possessing it, that Darwin's theories would be taxed with failure if they did not explain its origin. But climbing plants are found throughout the plant kingdom; and they are not descended from a common climbing ancestor, for they possess nothing in common except this one power to climb. “Plants,” he said, “become climbers in order, it may be presumed, to reach the light, and to expose a large surface of leaves to its action and to that of the free air. This is effected by the climbers with wonderfully little expenditure of organized matter in comparison with trees, which have to support a load of heavy branches by a massive trunk. Hence, no doubt, it arises that there are in all quarters of the world so many climbing plants belonging to so many different orders.” The very great advantage offered to the climber has acted as a powerful premium for the development of the capacity
1 Journal of the Linnean Society, 1865; Botany, Vol. IX. pp. 107, IoS.
wherever variation offered the materials out of which natural selection could produce it. The first thing to be established in proof of the derivation of climbing plants from nonclimbers was the existence of gradations in the power of climbing, and of intermediate stages between the different methods of climbing, — by twining of the stem, by leaf-stalks, and by tendrils; just as he connected the ocelli of the peacock's tail by a series of gradations with the more ordinary feather-markings of related birds. But another unknown element was the source of the variations upon which natural selection could work to produce climbers. In his arguments to prove his theories of descent and natural selection Darwin showed that variations do occur, and that when they occur natural selection will inevitably preserve the favorable and destroy the unfavorable. But he could do little or nothing in the direction of pointing out the cause of variations. He has been incessantly twitted about this by his opponents, especially because of the false notion that he ascribed to chance all variations whose causes were not known. Ignorance of the sources of variation is no obstacle whatever to belief in Darwin's theories; but this is true only when the species having a certain character in common are proved by many other characters to be descended from a common stock. In the case of climbing plants the same kind of variation must have occurred independently in all parts of the plant kingdom, or there must have been a common source or tendency which served as a starting point for the development of the power to climb. In his work on climbing plants, first published as a paper in the Linnean Journal,” he worked out in a masterly way the gradations in the power to climb, and between the different methods of climbing, showing that all are modifications of the method of climbing by twining of the stem. Near the end of his work he said, “We have seen how diversified are the movements of climbing plants. . . . They belong to many and widely different orders. . . . When we reflect on this wide serial distribution of plants having this power, and when we know that in some of the largest well defined orders, such as the Compositae, Rubiaceae, Scrophulariaceae, Liliaceae, etc., two or three genera alone out of a host of genera in each, have this power, the conclusion is forced on our minds that the capacity of acquiring the revolving power on which most climbers depend is inherent though undeveloped in almost every plant in the vegetable kingdom.”” It will be interesting to try to analyze the conditions under which he made this definite prediction. In his work on climbing plants he showed that the power to climb depends on two quite distinct powers: (1) the power of spontaneous circumnutation, and (2) sensitiveness to touch, and the consequent bending toward the side touched. Without the theory of descent, the question of the origin of the above mentioned sensitiveness could never have arisen at all. With the theory of descent, and with natural selection as a cause, and a belief in the existence of variations for it to work upon, it might have been possible to infer some general power or tendency in plants as the source of that sensitiveness to touch; but it was not done, and from what has been said elsewhere it is not likely that it would have been done, at least without great difficulty, from a knowledge of the highly specialized effects. Darwin said, “If we inquire how the petiole of a leaf, or the peduncle of a flower, or a branch, first becomes sensitive, and acquires the power of bending toward the touched side, we get no certain answer. Nevertheless, an observation by Hofmeister well deserves attention, namely, that the shoots and leaves of all plants, whilst young, move after being shaken; and it is almost invariably young petioles and young tendrils, whether formed of modified leaves or flower-peduncles, which move on being touched; so that it would appear as if these plants had utilized and perfected a widely distributed and incipient capacity, which capacity, as far as we can see, is of no service to ordinary plants.” " Darwin was in search of a source of the sensitiveness of plants, and Hofmeister had provided it by empirical observation. Darwin's relation to this explanation was exactly the same as it was in the discovery of the principle of natural selection. It will be seen that in the latter case he had studied very carefully the effects (adaptations) to be accounted for, and variations as the material upon which the unknown cause might act; then by accidental reading of Malthus the cause was presented to him, and he brought it and its effects into relation with each other by interpreting the latter as results of the action of the former. In the case of the sensitiveness to touch in plants,
1 Journal of the Linnean Society, 1865, Botany, Vol. IX, “On the Movements and Habits of Climbing Plants.”