In this same connection another question needs to be discussed. It is assumed by several botanists that in a normal plant the latent shoots or buds along the stem do not develop so long as the terminal shoots are growing, because the latter use up all the food material that is carried to that region. If the terminal bud is destroyed the lateral shoots then burst forth, in consequence, it is assumed, of the excess of food stuff that now comes to them. I do not believe that the phenomena can be so easily explained. If a piece of a plant is cut off, the leaves removed, and the piece suspended in a moist chamber and kept in the dark, the lateral buds at the apex will begin to develop. If we assume that the piece cannot develop any new food substance in the dark, then it contains just the same amount as it did while a part of the plant, and yet that amount is ample for the development of the lateral buds. Moreover, only the more apical buds develop; but if the piece is then cut in two, the apical buds of the basal piece, that had remained undeveloped, will now develop. How can this be explained by the amount of food substances in the piece? If it is assumed that in the normal plant the food substances flow only to the growing points, and the buds are out of the main current and fail in consequence to develop, it can be shown that this idea also fails to explain certain results. Vöchting has found, for example, that if an incision is made below a bud and the piece containing the bud be lifted up somewhat from the rest of the piece, remaining attached only at its anterior end, the bud will begin to develop. In this case the conditions preclude an accumulation of food substances in the piece, and the bud is even farther removed than at first from the main current, yet it begins to develop.

We shall find, I think, that the idea of food stuffs fails to explain some of the simplest phenomena, and while it need not be denied that under certain conditions the presence or accumulation of food materials may produce certain definite results, yet such food stuffs seem to play a very subordinate part as compared with certain other internal or innate factors.

CHAPTER V
REGENERATION AND LIABILITY TO INJURY

There is a widespread belief amongst zoologists that a definite relation exists between the liability of an animal to injury and its power of regeneration. It is also supposed that those individual parts of an animal that are more exposed to accidental injury, or to the attacks of enemies, are the parts in which regeneration is best developed, and conversely, that those parts of the body that are rarely or never injured do not possess the power of regeneration.

Not only do we find this belief implied in many ways, but we find this point of view definitely taken by several eminent writers, and in some cases carried so far that the process of regeneration itself is supposed to be accounted for by the liability of the parts to injury. In order that it may not appear that I have exaggerated the widespread occurrence of this belief, a few examples may be cited.

Réaumur in 1742 pointed out that regeneration is especially characteristic of those animals whose body is liable to be broken, or, as in the earthworm, subject to the attacks of enemies. Bonnet (1745) thought that such a connection exists as has just been stated, and that the animals that possess the power of regeneration have been endowed with germs set aside for this very purpose. He further believed that there would be in each animal that regenerates as many of these germs as the number of times that it is liable to be injured during its natural life. Darwin in his book on Animals and Plants under Domestication says: “In the case of those animals that may be bisected, or chopped into pieces, and of which every fragment will reproduce the whole, the power of regrowth must be diffused throughout the whole body. Nevertheless, there seems to be much truth in the view maintained by Professor Lessona[36] that this capacity is generally a localized and special one serving to replace parts which are eminently liable to be lost in each particular animal. The most striking case in favor of this view is that the terrestrial salamander, according to Lessona, cannot reproduce lost parts, whilst another species of the same genus, the aquatic salamander, has extraordinary powers of regrowth, as we have just seen; and this animal is eminently liable to have its limbs, tail, eyes, and jaws bitten off by other tritons.”

Lang, referring to the brittleness of the tails of lizards, points out that this is a very useful character, since the bird of prey that has struck at a lizard gets hold of only the last part of the animal to disappear under cover; the lizard escapes by breaking off its tail. The brittleness of the tail is, therefore, an adaptive character that has become fixed by long inheritance.

To this example may be added that of certain land snails in the Philippine Islands. The individuals of the genus helicarion live on trees in damp forests, often in great droves. They are very active, and creep with unusual swiftness over the stems and leaves of the trees. Semper has recorded that all the species observed by him have the remarkable power of breaking off the tail (foot) close behind the shell, if the tail is roughly grasped. A convulsive movement is made until the tail comes off, and the snail drops to the ground, where it is concealed by the leaves. Semper adds that in this way the snails often escaped from him and from his collectors, leaving nothing behind but their tails. The tail is said to be the most obvious part of the animal, and it is assumed that this is, therefore, the part that a reptile or bird would first attack.[37] Lang states that in this case external influences have produced an extraordinarily well-developed sensitiveness in the animal, so that it reacts to the external stimulus by voluntarily throwing off the tail. It would be, of course, of small advantage to be able to throw off the tail unless the power of regenerating the lost organ existed, or was acquired at the same time as the extreme sensitiveness that brings about the reaction. Lang does not state, however, explicitly that he believes the regenerative power to have arisen through the exposure of the tail of the lizard and the tail of the snail to injury, although he thinks that the mechanism by means of which these parts are thrown off has been acquired in this way. Several other writers have, however, used these same cases to illustrate the supposed principle of liability to injury and power of regeneration.

Weismann in his book on The Germ Plasm has adopted the principle of a connection between regeneration and liability to injury and has carried it much farther than other writers. We can, therefore, most profitably make a careful examination of Weismann’s position. His general idea may be gathered from the following quotation:[38] “The dissimilarity, moreover, as regards the power of regeneration in various members of the same species, also indicates that adaptation is an important factor in the process. In proteus, which in other respects possesses so slight a capacity for regeneration, the gills grow again rapidly when they have been cut off. In lizards again this power is confined to the tail, and the limbs cannot become restored. In these animals, however, the tail is obviously far more likely to become mutilated than are the limbs, which, as a matter of fact, are seldom lost, although individuals with stumps of legs are occasionally met with. The physiological importance of the tail of a lizard consists in the fact that it preserves the animal from total destruction, for pursuers will generally aim at the long trailing tail,[39] and thus the animal often escapes, as the tail breaks off when it is firmly seized. It is, in fact, as Leydig was the first to point out, specially adapted for breaking off, the bodies of the caudal vertebræ from the seventh onward being provided with a special plane of fracture so that they easily break into two transversely. Now if this capability of fracture is provided for by a special arrangement and modification of the parts of the tail, we shall not be making too daring an inference if we regard the regenerative power of the tail as a special adaptation, produced by selection, of this particular part of the body, the frequent loss of which is in a certain measure provided for, and not as the outcome of an unknown ‘regenerative power’ possessed by the entire animal. This arrangement would not have been provided if the part had been of no, or of only slight, physiological importance, as is the case in snakes and chelonians, although these animals are as highly organized as lizards. The reason that the limbs of lizards are not replaced is, I believe, due to the fact that these animals are seldom seized by the leg, owing to their extremely rapid movements.” Overlooking the numerous cases of the regeneration of internal organs that have been known for several years, and basing his conclusion on a small, unconvincing experiment of his own on the lungs of a few salamanders, Weismann concludes: “Hence there is no such thing as a general power of regeneration; in each kind of animal this power is graduated according to the need of regeneration in the part under consideration; that is to say, the degree in which it is present is mainly in proportion to the liability of the part to injury.”

After arriving at this conclusion the following admission is a decided anticlimax: “The question, however, arises as to whether the capacity of each part for regeneration results from special process of adaptation, or whether regeneration occurs as the mere outcomewhich is to some extent unforeseen—of the physical nature of an animal. Some statements which have been made on this subject seem hardly to admit of any but the latter explanation.” After showing that some newts confined in aquaria attacked each other, “and several times one of them seized another by the lower jaw, and tugged and bit at it so violently that it would have been torn off had I not separated the animals,”[40] and after referring to the regeneration of the stork’s beak, Weismann concludes: “Such cases, the accuracy of which can scarcely be doubted, indicate that the capacity for regeneration does not depend only on the special adaptation of a particular organ, but that a general power also exists which belongs to the whole organism, and to a certain extent affects many and perhaps even all parts. By virtue of this power, moreover, simple organs can be replaced when they are not specially adapted for regeneration.” The perplexity of the reader, as a result of this temporary vacillation on Weismann’s part, is hardly set straight by the general conclusion that follows on the same page: “We are, therefore, led to infer that the general capacity of all parts for regeneration may have been acquired by selection in the lower and simpler forms, and that it gradually decreased in the course of phylogeny in correspondence with the increase in complexity of organization; but that it may, on the other hand, be increased by special selective processes in each stage of its degeneration, in the case of certain parts which are physiologically important and are at the same time frequently exposed to loss.”