I might cite many more examples of localization of regenerative capacity, but will confine myself to one other, which seems to me particularly instructive, because it was first interpreted as an indication of the existence of an adaptive principle in the organism, a principle which always creates what is useful. I refer to the regeneration of the lens in the newt's larva.
G. Wolff, an obstinate opponent of the theory of selection, attempted to solve the same problem as I had before me in my experiments on the regeneration of the internal organs of newts, that is, he tried to answer the question whether organs which are never exposed to injury or to complete removal in the conditions of natural life, and which could not therefore have been influenced in this direction by the processes of selection, are nevertheless capable of regeneration. He extirpated the lens from the eye of Triton larvæ, and saw that in a short time it was formed anew, and from this he concluded that there was here 'a new adaptiveness appearing for the first time,' and that therefore adaptive forces must be dominant within the organism. The current theory of the 'mechanical' origin of vital adjustments seemed to some to be shaken by this, and the proclamation of the old 'vital force' seemed imminent. And in truth, if the body were really able to replace, after artificial injury, parts which are never liable to injury in natural conditions, and to do so in a most beautiful and appropriate manner, then there would be nothing for it but at least to regard the faculty of regeneration as a primary power of living creatures, and to think of the organism as like a crystal, which invariably completes itself if it be damaged in any part. But we have to ask whether this is really the case.
What makes the regeneration of the lens seem particularly surprising is the fact that in the fully formed animal it must arise in a manner different from that in which it develops in the embryo, that is, it must be formed from different cell-material. In the embryo it arises by the proliferation and invagination of the epidermic layer of cells to meet the so-called 'primary' optic vesicle growing out from the brain—a mode of development which cannot of course be repeated under the altered conditions in the fully developed animal. The reconstruction of the organ must therefore take place in a different way, and if the organism were really able, the very first time the lens was removed, to react in a manner so perfectly adapted to the end, and so to inspire certain cells, which had till then had a different function, that they could put together a lens of flawless beauty and transparency, we should have reason to suspect that nearly all our previous conceptions were erroneous, and to fall back upon a belief in a spiritus rector in the organism.
But the excision of the lens in these experiments was not by any means an unprecedented occurrence! It is true enough that newts in their pools are not liable to an operation for cataract, but it does not follow that the lens is never liable to injury, and could not therefore be adapted for regeneration. It can be bitten out along with the rest of the eye by water-beetles or other enemies, and as far back as the time of Bonnet and Blumenbach (1781) it was known that the eye of the newt would renew itself if it were cut out, given that a small portion of the bulb was left. But if this were removed the possibility of regeneration was at an end. Thus, before the first artificial excision of the lens, a regeneration-mechanism must have existed, by means of which the eye with its lens was reconstructed, and this depends on the characters of the cells of the eye itself—it is localized in the eye, and without the presence of a piece of eye-tissue no regeneration can take place. Is it then so especially remarkable that the lens should be renewed when it is artificially removed without the rest of the eye? The mechanism for its renewal is there, and is roused to activity whether the lens alone or other parts of the eye also be removed. We do not need, therefore, to assume the existence of a purposeful or adaptive force; it is more to the point to inquire where the regeneration-mechanism which suggests this inference is to be found.
A definite answer to this is given in a detailed experimental work recently published by Fischel. It corroborates what Wolff had already found, that the substance of the new lens develops from cells which cover the posterior surface of the iris, that is, from cells of the retinal layer of the eye. First, the margin of the pupil begins to react to the stimulus of the injury (extraction of the lens); its cells enlarge, become clear, while previously they were filled with dark pigment, and finally they proliferate. They thus form a cell-vesicle similar to the ectoderm-vesicle from which the lens arises in the embryo, and into this the already mentioned retina-cells from the posterior wall of the iris grow, elongate, and arrange themselves to form the so-called 'lens-fibres,' on whose form, arrangement, and transparency the function of the lens depends. This is marvellous enough, but not more marvellous than that a whole foot should grow on the cut stump of a newt's leg, or that a whole eye should arise from a residual fragment. Here, again, we do not know the processes which cause the arrangement of the cells and their often manifold locally-conditioned differentiations, in short, we do not know the essential nature of regeneration. But, in the meantime, we can endeavour to find out which cell-groups regeneration is bound up with in particular cases, so as to know where the vital particles, the 'determinants,' which condition regeneration, are placed by nature.
Fig. 99. Regeneration of the lens in the Newt's eye. A, section through the iris (J); from its margin and posterior (retinal) surface the primordium of a new lens (L) has developed after the artificial removal of the old one. B, section through the eye after duplicated regeneration of the lens (L) from two areas of the iris. Gl, vitreous humour. J, iris. C, cornea. R, retina. After Fischel.
In this case there can be no doubt on that point: they are the cells on the posterior wall and the margin of the iris. And it is certainly not the absence of the lens which gives rise to its renewal, as would necessarily be the case if it were due to the dominance of an adaptive force. If the lens, instead of being excised, be simply pressed back into the vitreous humour occupying the cavity of the eye, a new lens is developed all the same from the irritated margin of the pupil. And if by chance this margin has been irritated in two places while extraction of the lens was being performed, then two small lenses will develop (Fig. 99, B). Indeed, several may begin to develop at the posterior wall of the iris, although they do not attain to full development; mechanical irritation of any part of this cell-layer is responded to by the formation of lenses. This surely disposes of the 'mystical nimbus' which would dazzle us with a new force of life, always creating what is appropriate. We have before us an adaptation to the liability of newts' eyes to injury, which, like all adaptations, is only relatively perfect, since under the usual conditions of eye injury it gives rise to a usable lens, but under unusual conditions to unsuitable structures. It is exactly the same as in the case of animal instincts, which are all 'calculated' for the ordinary conditions of life, but, under unusual conditions, may operate in a manner quite unsuited to the necessary end. The ant-lion has the instinct to bore backwards into the sand, and he makes the same backward-pressing movements when placed on a glass plate into which he cannot force the tip of the abdomen. The same is true of the mole-cricket, which makes its usual digging movements with the forelegs even on a plate of glass. The wall-bee roofs over her cell when she has laid an egg in it, but she does so even if the egg be taken out beforehand, or if a hole be made in the bottom of the cell, so that the honey which is to serve the larva for food when it emerges from the egg runs out (Fabre). Her instinct is calculated for filling the cell once with honey, and once laying an egg in it, because such disturbances as we may cause artificially do not occur or occur very rarely in natural conditions. There are countless facts of this kind, for every instinct and every adaptation can, in certain circumstances, go astray and become inappropriate. This should be considered by those who still persist in opposing the theory of selection, for herein lies one of the most convincing proofs of its correctness. Adaptations can only arise in reference to the majority of occurrences, and variations which are only useful in an individual case must, according to the principle, disappear again. Adaptation always means the establishment of what is appropriate in an average number of cases.
Therefore the inappropriate reaction of the margin of the iris to an artificial double stimulus affords additional reason for regarding regeneration as an adaptive phenomenon. If it were the outcome of an adaptive force it could never be inappropriate; and if it were the operation of a general and primary power of the organism it would be exhibited by the nearly-related frog as well as by the newt. But, in the frog, extraction of the lens gives rise only to a sac-like proliferation of the cells of the iris margin, which form no transparent lens, but an opaque cluster of cells, which destroys vision altogether. It appears, therefore, that the frog no longer requires the power its ancestors possessed of regenerating a lost lens.