CHAPTER VI.

HOMOLOGIES OF THE ARTICULATE SKELETON.

We have taken the vertebrate skeleton first, only because this department is most familiar. But in reality, the most beautiful illustrations of essential identity of structure in the midst of infinite diversity of adaptive modification for different functions and habits of life, and therefore of common origin from a primal form, are found in the department of articulates. I use the old Cuvierian department _articulata_, rather than the more modern _arthropods_, because the former includes worms also. Now, whether worms should be thus included with arthropods, or deserve a whole department to themselves it matters not for our purposes. It is generally admitted that arthropods probably descended from marine worms. They all have the same general plan of skeletal structure. It will suit my purpose, therefore, to regard worms as the lowest form of jointed animals.

Here, then, we have an entirely different plan of structure--a different style of architecture and different mechanical principles of machinery. Instead of a skeleton within and muscles acting on the outside, we have the skeleton on the outside, and muscles acting from within. Instead of two cavities, a neural and visceral, the skeleton forms but one cavity, in which all organs are inclosed and protected. Instead of finding the nerve-axis on the dorsal aspect of the body, we find it on the ventral aspect.

Take any articulate animal, for example, a shrimp, a centiped, or a beetle. Cut it across the body, and look at the end (Fig. 26). We see a ring of bone (chitin) inclosing all the organs (nervous system _n_, blood system _b_, and visceral system _v_), and a pair of jointed appendages, perhaps legs, on each side. Now imagine these parts repeated in a linear series. The rings repeated make a hollow, jointed tube or barrel, the appendages repeated make a continuous row of appendages on each side. Now this is exactly what we actually find. The whole articulate skeleton is ideally made up of a series of such repeated rings and appendages, modified according to the position in the series, and the uses to which they are put. And then the whole articulate department is made up of such articulate animals again modified according to place in the scale of articulates. The modification in the lower forms is slight, and therefore the identity of the repeated parts is obvious; but as we go up the scale, and the number and complexity of the functions increase, the adaptive modification becomes greater and greater, until finally it so obscures the essential identity, that it requires the most extensive comparison in the taxonomic series and in the ontogenic series, to pick up the intermediate links and establish the fact of common origin. In a word, whether they so originated or not, it is certain that the structure of articulate animals is exactly such as would be the case if all these animals were genetically connected, and came originally from a primal form something like one of the lower crustaceans, or, perhaps, a marine worm.

It will be best to take an example from about the middle of the scale, where the two elements, viz., essential identity and adaptive modification, are somewhat evenly balanced, and both traceable with ease and certainty. Take, then, a cray-fish, a lobster, or a shrimp. This animal (Fig. 27) has twenty or twenty-one rings and pairs of jointed appendages. The rings are some of them diminished, some of them increased in size. Sometimes several are consolidated; sometimes several are partially or wholly aborted. The appendages are modified in shape and size, according to their position, so as to make them swimming-appendages (swimmerets), walking-appendages (legs), eating-appendages (jaws), and sense-appendages (antennæ). For example, in the abdominal region, or so-called tail, we have seven segments, all being perfect movable rings, each with its pair of jointed appendages, except the last, or _telson_. The appendages of the first ring (Fig. 28, B) are specially modified in the male as organs of copulation (B′). The next four pairs are modified for swimmerets (D′) and for use as holders of the eggs in the female. The appendages of the sixth ring (G) are broad and paddle-shaped, and, together with the telson or seventh ring (H), form the powerful terminal swimmer. Going, now, to the cephalo-thorax: in this either a large number of segments (thirteen or fourteen) are consolidated above to form the upper shell or carapace; or else, as is more probable, two or three of the anterior segments have enlarged and grown backward over, and at the expense of the others, to form this shell. At any rate, it is certain that the carapace is formed of the dorsal portions of a number of segments consolidated together. Below, however, the segments are all distinct, and have each its own pair of appendages. For example, going forward in this region, the five next pairs of appendages are greatly enlarged and very strong, and serve the purpose of locomotion. They are _walking-appendages_. The next two or three pairs are smaller and somewhat modified, but not so much as to obscure their essential similarity to legs. Like legs, they are many-jointed, and like legs, too, they have gills attached to them. They are called maxillipeds, or jaw-feet. They are used like hands to gather food and carry it to the mouth. They are _gathering-appendages_. Then follow three or four pairs still more modified, and used for mastication. They are called maxillæ and mandibles. They are _eating-appendages_. Then follow two pairs, long, many-jointed, with the same kind of curious hinge-joints, which we have in the legs, undoubtedly homologous with all the others, but used for an entirely different purpose, and specially modified for that purpose. They are the antennæ. They are delicate organs of touch and of hearing, for the ear is situated in the basal joint of the anterior pair. Last of all, there is still another pair, jointed and movable, on the ends of which are situated the eyes. These last three, therefore, are _sense-appendages_. Some writers make this last pair special organs, not homologous with appendages.

For the sake of greater distinctness, we give the whole series of these appendages in one of the higher forms, viz., the prawn (Palemon, Fig. 29, and in one of the lower forms, Nebalia, Fig. 30).

That these are really homologous parts is further shown by the fact that in the case of other crustaceans, such as limulus, the same appendages, i. e., the appendages of the same body segments, which in the cases before mentioned are used as feet, become swimmers, while the appendages corresponding to jaw-feet become walkers; and even what corresponds to antennæ or sense-appendages, may, as in branchippus, become powerful claspers. Finally, in all the lowest crustaceans, the identity is evident, because all the segments and their appendages are much alike in form and function (Fig. 31).

We have taken examples from near the middle of the articulate scale, because, as already stated, both the essential identity and the adaptive modifications are easily traced. If we go downward in the scale, the structure becomes more and more generalized, and the rings and appendages become more and more alike (Fig. 31), until in the most generalized forms we have only a series of similar rings, with similar pairs of appendages, except some necessary modifications to form the head and tail. This is well shown in the centiped (Fig. 32), and still better in marine worms (Fig. 33). In some marine worms the slight modification to form the head takes place under our very eyes. These often multiply by dividing themselves into two. When they do so, they make a new head and new tail by slight modification of segments and appendages (Fig. 33).

If, on the other hand, we go up the scale, we find adaptive modifications obscuring more and more the simple and obvious identity of parts, until finally the identity can not be recognized without extensive comparison in the taxonomic series and study of embryonic conditions. In crabs--which is a higher form than cray-fish--the tail or abdomen seems to be wanting, but is only very small and bent under the body and thus concealed. In all essential respects the structure is precisely like the cray-fish. In fact, in the embryo, we trace the one form into the other; for the crab is at first a long-tailed crustacean (Fig. 34).

Insects are the highest form of articulates. In these, therefore, we find the modification is still greater than in crustaceans, though even here the ring-and-appendage structure is plain enough in most cases.

One of the best evidences of high grade among animals is the gathering of the segments into distinct groups, and especially the distinctness of the _head_ as one of these groups. In worms and lower crustaceans there is no grouping at all, the skeleton being a continuous series of joints, only slightly modified at the anterior and posterior extremities. In the higher crustacea, and in spiders and scorpions, they are grouped into two regions, viz., cephalo-thorax and abdomen. In insects they are grouped into three very distinct regions--head, thorax, and abdomen. In insects, therefore, we find for the first time the head distinctly separated from the rest of the body. This is an evidence of high grade, because it shows the dominance of head-functions.

The insect, such, for example, as a grasshopper, consists of seventeen or eighteen segments (Fig. 35). Of these, four belong to the head, three to the thorax, and about ten to the abdomen. Those of the abdomen are all separated and movable; those of the thorax and head are more or less consolidated. The appendages of the head-segments become antennæ and jaw-parts, i. e., mandibles--maxillæ and labium; the appendages of the thorac-segments become legs (the wings are not homologous with appendages), while those of the abdomen are aborted. The steps of the gradual consolidation on the one hand, and the abortion on the other, may be traced in the embryo or larva--i. e., in the caterpillar or the grub of a bee or a beetle. In the caterpillar, for example, there is no grouping into three regions, there is no consolidation, and all the segments have appendages. Again, the almost infinite variety in the mouth-parts among insects, brought about by adaptive modifications for biting, for piercing, and for sucking, and yet the essential identity of all to the more simple and generalized structure of the grasshopper, is an admirable illustration of the same principle. But to dwell upon these minor points would carry us too far.

=Illustration of the Law of Differentiation.=--We have here, in the modifications of segments and appendages of articulates, an admirable illustration of the most fundamental law of evolution, viz., the law of differentiation. As we have already seen (page 21), perhaps the most beautiful and certainly the most fundamental illustration of this law is found in the development of cell-structure. Commencing in the lowest animals, and in the earliest embryonic stages of the higher animals, from a condition in which all are alike, the _cells_ as we go upward quickly diverge into different forms to produce different tissues and perform different functions. Here, then, we have a perfect example of essential identity and adaptive modification. It is the very best type of differentiation. So also skeletal _segments_, commencing, in the lowest articulates and in earliest embryonic stages of the higher, all alike, as we go upward in either series, begin immediately to diverge in various directions (divergent variation), taking different forms to subserve different uses. Here, again, therefore, is an illustration of the law of differentiation. Lastly, in the articulate department, commencing with the lowest forms and earliest embryonic conditions, and we may add earliest geological times, and going up either series from generalized forms very much alike, the _individuals_ are gradually differentiated into many special forms, in order to adapt them to the diversified modes of life actually found in nature. Thus cells, segments, individuals, are all alike affected by this most fundamental law.

We have taken our illustrations from only the two departments of vertebrata and articulata, because these are the most familiar to the reader, and also have been most carefully studied. We have shown that the general structure of all vertebrates is precisely what it would be if they all had come from one primal vertebrate form, and that of all articulates what it would be if all had come from one primal articulate form. The only _natural_ explanation, and, therefore, the only scientific explanation of this, is that _they were really thus derived_. The same kind of evidence may be drawn from the study of other departments, but to pursue the subject any further in this direction would carry us beyond the limits which we have assigned. We desire only to explain the nature, not to give all, of the evidence. The examples given will be sufficient for the purposes of illustration. The whole proof is nothing less than the whole science of comparative anatomy.

Vertebrates, then, were derived from a primal vertebrate, articulates from a primal articulate, and so for other departments. But whence were these _primals_ derived? Are there any intermediate links between, any deeply concealed common plan of structure underlying these primary groups, showing a common origin? It must be confessed that, in their _mature_ condition, there seems to be but little evidence of such. These primary groups seem to be built on different plans, to be fundamentally of different styles of architecture. Therefore Darwin, in the true spirit of inductive caution--that true scientific spirit which keeps strictly within the limits of evidence--commences with four or five distinct primal kinds, from which by divergent variation all animals were descended. Nevertheless, the truly scientific biologist must ever strongly incline to believe that these also came from some _primal animal_, and even that both animals and plants were derived from some primal form of _living thing_; that as, in the taxonomic series, the animal and vegetal kingdoms in their lowest forms merge undistinguishably into one another; as in the ontogenic series the animal and plant germ are one, so also in the phylogenic series the earliest organisms were simply living things, but not distinctively animal nor vegetal. Science, therefore, whose mission is to trace origins as far back as possible, must ever strive to find connecting links between the primary groups. Some such have been supposed to have been discovered. Some find the origin of vertebrates among the molluscoids (ascidians); some find the origins of both vertebrates and articulates among marine worms (annelids). This point is still too doubtful to be dwelt upon here. It may be that we seek in vain for such connecting links among existing forms. It may well be that the point of separation of these great primary groups (unless we except vertebrates) was far lower even than these low forms. Both phylogeny and ontogeny seem to indicate this. In the earliest fauna known, the primordial (for if there was life in the archæan it was not yet differentiated into a fauna), all the great departments, except the vertebrates, seem to have been represented. In embryonic development, too, the point of connection or even of similarity, between the great departments, is found, as we shall see hereafter, only in the earliest stages--i. e., lower down than any but the lowest existing forms, viz., the protozoa.