Our analysis leads to the conclusion that we can neither account for the phenomenon of autotomy as due to internal causes alone in the sense of its being a general property of protoplasm, nor to an external cause, in the sense of a reaction to injury or loss from accident. There would seem then only one possibility left, namely, that it is a result of both together, or in other words, a process that the animal has acquired in connection with the conditions under which it lives, or in other words, an adaptive response of the organism to its conditions of life.
We are not, however, able at present to push these questions farther, for, however probable it may seem that animals and plants may acquire characteristics useful to them in their special conditions of life, and yet not of sufficient importance to be decisive in a life and death struggle, still we cannot, at present, state how this could have taken place in the course of evolution. For, however plausible it may appear that the useful structure has been built up through an interaction between the organism and its environment, we cannot afford to leave out of sight another possibility, viz. that the structure or action may have appeared independently of the environment, but after it appeared the organism adopted a new environment to which its new characters made it better suited. If the latter alternative is true, we should look in vain if we tried to find out how the interaction of the environment brought about the adaptation. The relation would not be a causal one, in a physical sense, but the outcome of a different sort of a relation, viz. the restriction of the organism to the environment in which it can remain in existence and leave descendants.
CHAPTER IX
GRAFTING AND REGENERATION
By uniting parts of the same or different animals, or of plants, there is given an opportunity of studying a number of important problems connected with the regeneration of the grafted parts. Trembley’s experiments in grafting pieces of hydra are amongst the earliest recorded cases of uniting portions of different animals, although in plants the process of grafting has been long known.[77] Trembley found that if a hydra is cut in two, the pieces can be reunited by their cut-surfaces, and a complete animal results. No regeneration takes place where the union has been made. He also succeeded in uniting the anterior half of one individual with the posterior half of another individual, and again produced a single individual. He failed to obtain a permanent union between different species.
More recently, Wetzel has carried out a number of different experiments in uniting pieces of hydra. He found that if two hydras are cut in two, the two anterior pieces may be united by the aboral cut-surfaces ([Fig. 46], B), and the two posterior pieces may also be united by their oral cut-surfaces ([Fig. 46], A). The fusion of these “like-ends” takes place as readily as when unlike ends are brought in contact, as in Trembley’s experiments. Subsequently, however, regenerative changes take place. When, for instance, two anterior pieces are united by their aboral ends, there develop after two or three days one or two outgrowths, at or near the line of union, that become new feet, and the two individuals may subsequently separate. When two posterior pieces are united by their oral surfaces, a double circle of tentacles generally develops, one on each side of the line of union. The pieces then pinch apart and produce two hydras.[78] In another experiment the head and a part of the foot were cut from a hydra, and the head was turned around and grafted by its aboral surface upon the aboral surface of the middle piece. Another animal was cut in two in the middle, and the posterior half was grafted by its oral end to the oral end of the middle piece. In this way a new, artificial individual was made, as shown in [Fig. 46], C, with the middle part of the body in a reverse direction as compared with the orientation of the two end-pieces.[79] The union of the three pieces was so perfect that not even a swelling or a constriction indicated the places of fusion. After six days a normal bud appeared at the region of union of the posterior and middle pieces, that gave rise to a new hydra, which separated after a few days. The compound animal was healthy and ate many daphnias. It was kept under observation for twenty-four days, and appeared normal, giving off several more buds.
Fig. 46.—A. Two posterior pieces of hydra united by their oral ends. B. Two anterior pieces of hydra united by their aboral ends. C. A “long hydra” made by uniting three pieces; the middle piece reversed. D. After Peebles. Two posterior pieces of brown hydra united by oral ends, and one cut off near union. A new anterior end developed from the cut, aboral surface. F. After Peebles. Union of a nutritive and a protective polyps of hydractinia. Subsequently former cut off at line, 1-1. E. Union of two posterior pieces of hydra by oral ends. Subsequently one piece cut off at line, 2-2. E¹. New head regenerated in region of union, and a foot from aboral cut-end. E², E³. Fusion of two parts with a single hydra.
In other experiments of this same sort a foot generally developed where the two aboral surfaces came together, and the head-end separated from the rest of the piece. In another case a mouth and tentacles appeared at the place at which the oral ends had united.
In a different kind of experiment, the anterior ends of two hydras were cut off and united by their aboral surfaces; then one of the components was cut in two, just back of the circle of tentacles. After five days two short, hook-like processes appeared at the cut, oral end. They produced a foot, by means of which the animal fixed itself. In this case it will be seen that a foot developed from an oral end. The result might not in itself be considered sufficient to show whether the development of a foot at the oral end of a piece is due to the influence of the other component, or is simply a case of heteromorphosis having no connection with the presence of the other component. Since heteromorphosis has never been observed in isolated pieces of hydra, the probability is that the result is in some way connected with the presence of the other component. Peebles has made a number of experiments, in which special attention was paid to this point. Fifteen anterior pieces were united in pairs by their aboral cut-surfaces, and then one component was cut in half, leaving an exposed oral end. Out of this number five pieces formed a new head at the cut-surface, and the pieces became attached by a foot, that developed at the region of union. Two others did not regenerate at the cut-surface, but became fixed as before, and neither regenerated nor became fixed at the cut-end. Three became attached at the cut, oral surface, but none of these developed a characteristic foot. The result shows, nevertheless, that some influence was present that inhibited the development of a mouth and tentacles at the oral cut-end, since these always develop in isolated pieces. In another series of experiments posterior ends were united by their oral surfaces, and then one of the two pieces was cut in two ([Fig. 46], E). A new hypostome and tentacles developed at the region of union, and a foot at the aboral cut-surface, as shown in [Fig. 46], E¹. An organism, with one mouth and a circle of tentacles, and two bodies and two feet, resulted. The bodies soon began to fuse together ([Fig. 46], E²) into a single one, and when the fusion had extended to the region of the feet, they also fused into a single structure ([Fig. 46], E³), so that a single hydra was produced.