The Evolution Theory, Vol. 2 of 2

by August Weismann

Hardly any other vertebrate has attained such celebrity on account of its high regenerative capacity as the water-newt, species of the genus Triton. It can regrow not only its tail, but the legs and their parts if they are cut off. Spallanzani saw the legs grow six times, after he had cut them off six times. In the blind newt (Proteus) of the Krainer caves, a near relative of the common newt, the leg regenerated only after a year and a half, although the animal stands on a lower stage of organization than the newt, and thus should rather replace lost parts more easily. But Proteus lives sheltered from danger in dark, still caves, while Triton is exposed to numerous enemies which bite off pieces from its tail or legs; and the legs are its chief means of locomotion, without which it would have difficulty in procuring food. It is different with the elongated eel-like newt of the marshes of South Carolina, Siren lacertina. This animal moves by wriggling its very muscular trunk, after the manner of an eel, and in consequence of the disuse of its hind legs it has almost completely lost them. Even the fore-legs have become small and weak, and possess only two toes, and these do not regrow if they are bitten off, or only do so very slowly.

Earthworms are exposed to much persecution; not only birds, such as blackbirds and some woodpeckers, but, above all, the moles prey upon them, and Dahl has shown that moles often lay up stores of worms in winter which they have half crippled by a bite, while even Réaumur knew that moles frequently only half devoured earthworms. It was thus an obvious advantage to earthworms that a part of the animal should be able to regrow a whole, and accordingly we find a fairly well-developed regenerative capacity among them. But it varies greatly in the different species, and it would be interesting if we knew the conditions of life well enough to be able to decide whether the faculty of regeneration rises and falls in proportion to the dangers to which the species is exposed. Unfortunately we are far from this as yet; we only know that, in the common earthworms of the genera Lumbricus and Allolobophora, the faculty of regeneration is still very limited, for at most two worms, and sometimes only one, can develop from an animal cut into two pieces. Cutting into a greater number of pieces does not yield a larger number of worms, but usually only one, and often none at all.

This corresponds to the behaviour of their enemies, which may often bite off a piece or tear it away when the worm attempts to escape, but never cut it up into pieces. The regenerative capacity is more highly developed in the genus Allurus, more highly still in the worms of the genus Criodrilus which lives in the mud at the bottom of lakes, and most highly of all in the genus Lumbriculus which lives at the bottom of small ponds. Long ago Bonnet cut up a specimen of Lumbriculus into twenty-six pieces, of about two millimetres in length, and he observed most of these grow to complete worms again. His experiments have often been repeated in recent times, and have been extended and made more precise in many ways. Von Bülow was able to get whole animals from pieces consisting of from four to five somatic segments, and with eight or nine segments he almost invariably succeeded. A Lumbriculus which he had cut into fourteen pieces, one of which only measured 3.5 mm. in length, gave rise to thirteen complete worms with head and tail; only one piece perished.

These worms have little enemies with sharp jaws which may gnaw at them behind or before but cannot swallow them whole. Lyonet, famous for his analytic dissection of the wood-caterpillar (Cossus ligniperda), observed when he was feeding the larvæ of dragon-flies with these Lumbriculid worms that 'the anterior end of some whose posterior end had been gnawed away by the larvæ continued to live on the ground.' We can thus understand why a high power of regeneration is of use to these worms, and at the same time why it is advantageous to them to contract so that they break in pieces on very slight irritation, but to this we shall refer again.

The very diverse potency of the faculty of regeneration in animals belonging to the same small group, and nearly, if not quite, at the same level of organization, seems to show clearly that we have here to do with adaptation to different conditions of life, although we cannot demonstrate this in detail. It would certainly be erroneous to regard the conditions of life as uniform, since the worms in question not only live in different places—in the earth, in mud, or in water—and are thus exposed to different enemies, and since they may also be quite different in regard to size and speed, in means of defence, and possibly also of defiance, as is indeed in some measure demonstrable.

We meet with the same thing in a group of still smaller worms, Rösel's 'water-snakelets,' species of the genus Nais. These, too, behave in a variety of ways in the matter of regeneration, for while many species, such as Nais proboscidea and Nais serpentina will, if cut into two or three pieces, become two or three worms respectively, Bonnet expressly mentions an unnamed species of Nais which does not bear cutting up at all, and even dies if its head be cut off.

Thus neither the degree of organization nor the relationship alone determines the strength of the regenerative capacity. And as nearly related species may behave quite differently in this respect, so also do the different parts of one and the same animal; and here, too, the strength of the capacity seems to depend on the more frequent or rarer injury of the relevant part and on its importance in the maintenance of life. Let us take a few examples.

Parts which, in the natural life of the animal, are never injured, show in many cases no power of regeneration. This is so in regard to the internal parts of the newt, whose regenerative capacity is otherwise so high. I cut half or nearly the whole of a lung away from newts anæsthetized with ether; the wound closed, but no renewal of the organ took place. The same thing happened when a piece of the spermatic duct or of the oviduct was cut away. It is true that the kidney enlarges in higher animals when a piece has been cut out, by the proliferation of the remaining tissues, but that is a mere physiological substitution, evoked by the increased functional stimulus, due to the accumulation in the blood of the constituents of the urine. Such substitution depends on the growth of parts already existing, and it occurs in man when one kidney is removed, for the other, as is well known, may then grow to double its normal size. This is mere hypertrophy of the part that is left, it is not regeneration in the morphological sense, and it is not comparable to the re-formation of a cut-off leg in the salamander, or of a head in the worm, where the growth is not a mere increase of the remaining stump, but a new formation. It would be regeneration if a new kidney developed from the remnants of the kidney-tissue, or, in the liver, if new lobes grew in place of those which were cut off. But neither of these things happens, and, as far as I am aware, nothing of the kind has ever been observed, nothing more than new formation of liver-cells through increase of existing ones; that, however, is not regeneration in the morphological sense.

I have referred to the slight power of regeneration possessed by the blind Proteus in regard to its legs or tail, and I connected this with the absence of enemies in its thinly peopled cave-habitat. But the same animal can regenerate its gills when these are bitten off, and this is probably associated with the habit that Proteus has, in common with other newts with external gills, of nibbling at its neighbour's gills. Thus, the power of regenerating the gills was retained even when the animals migrated to the quiet caves of Krain, and were thus secured from the attacks of other enemies.