It was Herbert Spencer, the English philosopher, who first brought the argument of co-adaptation into the field against my view of the non-inheritance of functionally acquired modifications. He pointed out that many, if not, indeed, most modifications of bodily parts, to be effective, implied further changes, often very numerous, in other parts, and these latter must therefore have changed simultaneously with the part which was being changed under the control of natural selection; this, however, is only conceivable as due to an inheritance of the changes caused by use, since a simultaneous alteration of so many parts through natural selection would be impossible. If, for instance, the antlers of our modern stag were to grow to the size of those of the Giant Stag of the Irish peat-bogs, which measured over ten feet across from tip to tip, this would mean—as has already been shown—a simultaneous thickening of the skull, and to bear the heavy burden, a strengthening of the ligamentum nuchæ, of the muscles of the neck and back, of the bones of the legs and their muscles, and, finally, of all the nerves supplying the muscles; and how could all this happen simultaneously with, and in exact proportion to the growth of the antlers, if it depended—as natural selection assumes—on chance variations of all these parts? What if the appropriately favourable variation in one of these organs did not occur? A harmonious variation of all the parts—bones, muscles, nerves, ligaments—which unite in a common activity, is an inadmissible assumption, because, in many cases, such co-operating groups of organs have in the course of evolution developed in opposite directions. In the giraffe, for instance, the fore-legs are longer than the hind-legs, which is the reverse of what obtains in the majority of ruminants; in the kangaroo the hind-legs, on the contrary, have developed to a disproportionate size, while the fore-legs have degenerated into relatively small grasping arms. Co-operating parts, like the fore and hind limbs, may thus follow opposite paths of evolution; their variations need not always be directed to the same end.
The difficulty presented by these so-called co-adaptations or harmonious correlations cannot be denied, and we must also admit that, if the results of exercise were inherited, the explanation of the phenomenon would, in many cases—but not, indeed, in all—be easy, because the adaptation of the secondarily varying parts in each individual life would correspond exactly to the altered function of the part, and would be transmitted to the descendants, and in them would again be subject to such a degree of variation, according to the principle of histonal selection, as might be conditioned by the further progress of the primary variation. The simplicity of the explanation is striking, if only it were at the same time correct! But there are whole series of facts, or rather of groups of facts, which prove that the causes of co-adaptation do not lie in the inheritance of functional modifications, and this must be recognized, even though we may not yet be in a position to state the causes of co-adaptation, and to say whether natural selection suffices to explain it or not.
I must first point out that co-adaptations occur not only in actively, but also in passively functioning parts. Very numerous instructive examples are to be found among the Arthropods, whose whole skeleton belongs to this category. It has been objected that this is not wholly passive, but that, like the bones of vertebrates, it is stimulated by the contraction of the muscles and incited to functional reaction, and that it thickens at places where strong muscles are inserted, and becomes or remains thin where it is not exposed to any strain from the muscles. But this is not the case, for the chitinous skeleton can only offer resistance to the muscular contractions when it is no longer soft, as it is immediately after it is secreted. As soon as it has become hard, it can no longer be altered, and can at most be worn away externally by long use. The proof of this lies in the necessity for moulting, which is indispensable to all Arthropods as long as they continue to grow, but does not occur later. Every one who has followed the growth of an insect or a crustacean knows well that the moultings or ecdyses are often accompanied by great changes, and hardly ever occur without some slight changes in the form of the body, especially of the limbs, with their teeth, bristles, spines, and so on. These new or transformed parts are formed before the throwing-off of the old chitinous shell, and under its protection, and they are brought about by an elaboration or transformation of the living soft matrix of the skeleton, the hypodermis, which consists of cells, and is the true skin. They must thus have arisen in the ancestors of our modern Arthropods in the same way, that is, not by a gradual modification during use, but by a slight sudden transformation before use. The steps in the transformation may have been very small, a bristle may have become a little longer in the second stage of life than it was in the first, or instead of five bristles a particular spot may bear six in the second or third stage of life; but the variations in the phyletic development must always be caused by germ-variations which effect from within the variation in the relevant stage of development. But the part which has varied can only function after it has become firm and immodifiable.
If these circumstances be kept clearly in mind, they furnish a quite overwhelming mass of proof against the views of the Lamarckians.
Furthermore, it is not even true that the thickest parts of the external skeleton are those at which the muscles are inserted. The wing-covers of beetles offer the best proof to the contrary, for there are no muscles at all in them, yet they are, in many species, the hardest and thickest part of the whole chitinous coat of mail. The reason is not far to seek; they protect the wings and the soft skin of the back, which lies concealed beneath them, and the muscles are inserted in this!—a relation which can be explained only by its suitability to the end, and not as due to any direct effect.
When we remember the origin—which we have just described—of the external skeleton from the soft layer of cells underneath it, the thickness of the chitinous skeleton, which is very different at different places in the same animal, but always adapted to its end, furnishes a case of co-adaptation in parts which have a purely passive function. The thickened part cannot be due to the insertion of a muscle, but it is always there in advance, from internal causes, so that the muscle finds sufficient resistance. Close to it there may lie, perhaps, the edge of a segment, and at this spot the chitinous skeleton becomes almost suddenly thinned to a joint membrane capable of being bent or folded, not because there was no pull from the muscles at this spot, but in order that the two segments may be connected movably. Thus, nowhere in the whole body of the Arthropod can the adaptation of the skeleton, in regard to thickness and power of resistance, be regulated by function itself, but only by processes of selection which imparted to each spot the thickness it required, in order to be effective in its function, whether that be offering resistance to the strain of the muscles, or giving suppleness to a joint, or affording the necessary hardness for biting the prey, or for boring into wood or earth, or merely for protecting the animal from external injuries.
Fig. 91 (repeated). Hind-leg of a
Grasshopper (Stenobothrus protorma), after
Graber. fe, femur. ti, tibia. ta, tarsal
joints. schr, the stridulating ridge.
There are, however, many individual functions of the Arthropods the exercise of which depends on the simultaneous change of several skeletal parts; as, for instance, many of the 'singing' or vocal apparatuses in insects. In quite recent times such vocal organs have been discovered in ants, in which they consist of a small striated region on the surface of the third abdominal segment, and a sharp ridge on the segment in front; the latter is rubbed against the former by the movements of the two segments. Quite a similar 'stridulating organ' has long been known in the bee-ant (Mutilla), and the whistling sound produced by it is easily heard by our ears; moreover August Forel has heard it in the large wood-ant (Camponotus ligniperdus), and has described it as an 'alarm-signal,' which the animals give each other on the approach of danger—an observation which has recently been confirmed by Wasmann and extended by Robert Wroughton in regard to Indian ants. All these arrangements for producing sound depend always on two organs, of which one resembles the bow, the other the strings of a violin; the one is of no value without the other, and they must therefore have developed simultaneously, yet they cannot have arisen through use, and the inheritance of the results of use, because they are both dead chitinous parts, which are never strengthened by rubbing against each other with the movements of the abdomen, but are rather worn away.
The same is true of the chirping organs of grasshoppers, beetles, and crickets; in all cases they consist of two different parts, which together produce a sound, and which therefore must have arisen simultaneously, and the origin of which cannot be referred to the inheritance of the results of exercise, but rather to selection. It is thus possible that co-adaptation of at least two parts may take place even when the hypothetical Lamarckian principle is altogether excluded.