Regeneration

by Thomas Hunt Morgan

It has been observed[76] that if the leg of tarantula is cut off at any other place than at the coxa, the animal bites off the wounded leg with its jaws down to the coxa. In other spiders this does not occur, although Schultz has observed that when the legs are lost under natural conditions they are found broken off in most cases at the coxa. Schultz has also found that the legs regenerate better from this region than from any other. It would be rash, I think, to conclude without further evidence that the habit of tarantula to bite off a wounded leg down to the coxa has been acquired in connection with the better regeneration of the leg at this place. It is possible that the injury may excite the animal to bite off the leg as far as possible, which might be to the coxal joint. It would certainly be very remarkable if this spider had acquired the habit in connection with the better regeneration of the leg at the base, since the leg can presumably also regenerate at any level, as in the epeirids.

In this same connection I may record that in the hermit-crab I have often observed that when a leg is cut off outside of the breaking-joint, if the leg is not thrown off at once, the claws of the first legs catch hold of the stump and, pulling at the leg, offer sufficient resistance for the leg to break off at the breaking-joint. I cannot believe that this instinct has anything to do with the better regeneration of the leg at the coxal joint, however attractive such an hypothesis may appear.

THEORIES OF AUTOTOMY

A number of writers have pointed out that under certain conditions it is an obvious advantage to the animal to be able to throw off a portion of the body and thereby escape from an enemy. It has been suggested that if a crab is seized by the leg, the animal may save its life at times at the expense of its leg; and since the crab has the power of regenerating a new leg, it is the gainer in the long run by the sacrifice. The holothurian, that ejects its viscera, has been supposed to offer a sufficient reward to its hungry enemy, and escapes paying the death penalty, at the expense of its digestive tract. Thus, having shown that the process of autotomy is a useful one, it is claimed that it must have been acquired through a process of natural selection! An equally common opinion is that autotomy is an adaptation for regeneration, since in certain cases, as in that of the crab’s leg, better conditions for subsequent regeneration occur at the breaking-joint than when the amputation takes place at any other region. Since less bleeding takes place when the crab’s leg is thrown off at the breaking-joint, and since the wound closes more quickly when the arm of the starfish is lost at the base, it is assumed that we have in both cases an adaptation to meet accidents, and that the adaptation has been acquired by natural selection.

A consideration of these questions involves us once more in a discussion of the theory of natural selection. An attempt has been made in another place (pages 108-110) to show that we are not justified in assuming that because a process is useful, therefore it must have been acquired by means of natural selection. Even if it were granted that the theory of natural selection is correct, it does not follow that all useful processes have arisen under its guidance. We may, therefore, leave the general question aside, and inquire whether the process of autotomy could have arisen through natural selection (admitting that there is such a process, for the sake of the present argument), or whether autotomy must be due to something else.

If we assume that the leg of some individual crayfishes or crabs, for example, broke off, when injured, more easily at one place than at another, and that regeneration took place as well, or even better, from this region than from any other, and if we further assume that those animals in which this happened would have had a better chance of survival than their fellows, then it might seem to follow that in time there would be more of this kind of animal that survived. But even these assumptions are not enough, for we must also assume that this particular variation was more likely to occur in the descendants of those that had it best developed, and that amongst those forms that survived, some had the same mechanism developed in a still higher degree, and, the process of selection again taking place, a further advance would be made in the direction of autotomy. This, I think, is a fair, although brief, statement of the conventional argument as to how the process of natural selection takes place. But let us look further and see if the results could be really carried out in the way imagined, shutting our eyes for the moment to the number of suppositions that it is necessary to make in order that the change may occur. It will not be difficult, I believe, to show that even on these assumptions the result could not be reached. In the first place, the crabs that are not injured in each generation are left out of account, and amongst these there will be some, it is true, that have the particular variation as well developed as the best amongst those that were injured, and others that have the average condition, but there will be still others that have the possibilities less highly developed, and the two latter classes will be, on the hypothesis, more numerous than those in the first class. The uninjured crabs will also have an advantage, so far as breeding and resisting the attacks of their enemies are concerned, as compared with those that have been injured, and in consequence they, rather than the injured ones, will be more likely to leave descendants. Even if some of those that have been injured, and have thrown off the leg at the most advantageous place, should interbreed with the uninjured crabs, still nothing, or very little, can be gained, because, on Darwinian principles, intercrossing of this sort will soon bring back the extreme variations to the average.

The process of natural selection could at best only bring about the result provided all crabs in each generation lose one or more of their legs, and amongst these only the ones survive that break off the leg at the most advantageous place; but no such wholesale injury takes place, as direct observation has shown. At any one time only a small percentage, about ten per cent, have regenerating legs, and as the time required completely to regenerate a leg, even in the summer, is quite long, this percentage must give an approximate idea of the extent of exposure to injury. It is strange that those who assert off-hand that, because autotomy is a useful process, therefore it must have been acquired by natural selection, have not taken the pains to work out how this could have come about. Had they done so, I cannot but believe they would have seen how great the difficulties are that stand in the way.

A further difficulty is met when we find that each leg of the crab has the same mechanism. If we reject as preposterous the idea that natural selection has developed in each leg the same structure, then we must suppose that a crab varies in the same direction in all its legs at the same time; and if this is true it is obvious that the principle of variation must be a far more important factor in the result than the picking out of the most extreme variations. The same laws that determine that one individual varies in a useful direction farther than do other individuals may, after all, account for the entire series of changes. If it be replied that natural selection does not take into account the causes of the differences of individual variation, this is to admit that it avowedly leaves out of account the very principles that may in themselves, and without the aid of any such supposed process as natural selection, bring about the result. The Lamarckian principle of use and disuse does not give an explanation of autotomy, since the region of the breaking-joint is not the weakest region of the leg, or the place at which the leg would be most likely to be injured.

We cannot assume autotomy to be a fundamental character of living things, since it occurs only under special conditions, and in special regions of the body. While it might be possible to trace the autotomy of the legs of the crustacea, myriapods and insects, to a common ancestral form, yet this is extremely improbable, because the process takes place in only a relatively few forms in each group. The autotomy of the wings of white ants that takes place along a preëxisting breaking-line must certainly have been independently acquired in this group. The breaking off of the end of the foot in the snail helicarion is also a special acquirement within the group of mollusca.

Bordage has suggested that the development of the breaking-joint at the base of the leg of phasmids has been acquired in connection with the process of moulting. He has observed that during this period the leg cannot, in some cases, be successfully withdrawn through the small basal region; and hence, if it could not break off, the animal would remain anchored to the old exoskeleton. It escapes at the expense of losing its leg. The animal, having acquired the means of breaking off its leg under these conditions, might also make use of the same mechanism when the leg is held or injured, and thereby escape its enemy. The fact that the crayfish has a breaking-joint only for the large first pair of legs would seem to be in favor of this interpretation, but the crab has the same mechanism for the slender walking legs, that one would suppose could be easily withdrawn from the old covering. It should also be remembered that we do not know whether the breaking-joint at the base of the leg of the crab and of the crayfish would act at the time when the leg is being withdrawn from the old exoskeleton, unless the leg were first injured outside of the joint.