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know the nature of the action of gravity on Newton’s apple. It was treated as a dead substance, like a stone, and gravity acted upon it as upon all other ponderable matter. But when it had fallen to the earth, had decayed, and one of its pips began to grow, the action of gravity began to be manifested in a quite different and very peculiar fashion. It has been ascertained by a series of ingenious experiments that gravity is the force which obliges the roots of a plant to sink downwards into the earth. This does not, of course, mean that the roots are drawn downwards by attraction of the earth, but that the pull of gravitation gives a certain stimulus to the cells concerned which makes them grow in that direction. Precisely the same stimulus communicated to the cells of the stem has the very opposite effect—these it causes to grow upright into the air and light. Thus the roots are, as it is termed, positively, and the stems negatively, geotropic. The substance of the root cells and of the stem cells is the same, the stimulus is the same, but the effects on growth agree in only one point, that they are respectively what the plant requires them to be. There is no doubt that if a species of plants were placed in such a position that it would serve them for the roots to grow upwards, then upward-growing roots would eventually be evolved; in fact, this is actually the case in the lateral underground roots of certain mangroves which rise to the surface and become modified as breathing organs, and in the aerial roots of various orchids, etc.52 When a change of habitat takes place calling for new developments of structure to meet new conditions, these developments are not, as a matter of actual observation, found to be mechanically ‘selected’ from a mass of random movements and modifications of tissue—they reach their goal, it is true, by a series of gradual approximations, but the goal is in sight from the beginning. In other words, adaptability is a fundamental character of life. Hence the fact that multicellular organisms which cannot, as a whole, fuse with others, adapt themselves to these conditions by the allotment of special cells for that purpose; while, again, the production of multicellular organisms is itself an adaptation to Nature’s need for the higher organization of life.

      “The botanist Reinke,” writes Weismann, “has recently called attention once again to the fact that machines cannot be directly made up of primary physico-chemical forces or energies, but that, as Lotze said, forces of a superior order are indispensable, which so dispose the fundamental chemico-physical forces that they must act in the way aimed at by the purpose of the machine.... Organisms also [according to Reinke] are machines which perform a particular and purposeful kind of work, and they are only capable of doing so because the energies which perform the work are forced into definite paths by superior forces; these superior forces are thus ‘the steersmen of the energies.’”53

      Weismann admits that there is “undoubtedly a kernel of truth in this view,” but he is content with this perfunctory acknowledgment. His main efforts are devoted to the substitution of fortuitously developed “constellations” of molecular energy for any force which can be deemed to have the slightest tincture of intelligence or purpose. “In our time,” as he writes, “the great riddle has been solved—the riddle of the origin of what is best suited to its purpose without the co-operation of purposive forces.” The nature of the proposed solution can be best described and discussed in another chapter, when we shall be in a position to consider it in relation to the whole history of organic development from its origin in protoplasmic life to the evolution of species in plants and animals.

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