extremity of the body. Soon tentacles are developed from the head portion, the body becomes segmented, and the tentacle which, under the name of Operculum (Fig. 23,0), is destined to form a stopper to the mouth of the tube, may likewise be discerned. At this stage, with its segmented body, the young tube-dwelling worm resembles the permanent condition of its free-living neighbour of the sand (Fig. 21). Hence, when we discover that the tube-dweller finally secretes a tube, and lodges its body therein, becoming a stationary form, we conclude, rationally enough, that both kinds of worms have arisen from one common stock, and that the tube-dwellers represent the more modified race of the two groups; whilst they likewise may be regarded as "degraded" forms when compared with their freeliving neighbours.

We have thus had presented to view a series of developments extending from those of the molluscs, through the "lampshells," and finally ending with that of the worms themselves. Is there evidence at hand to show that something more than a theoretical conception of the connection between these apparently dissimilar forms is a warrantable thought? The answer to such a question depends on the credence we place on what development teaches. If the truth of the axiom that "development repeats descent" be not admitted, it is worse than useless to invite coin

Unless the

parison between the larva of a chiton and that of a worm. mind has been prepared to discover in development the shifting and progressive past history of a species, there can be no benefit of an intellectual kind in comparing the likeness of the young brachiopod with the early stages of the worm. But, conversely, when it is admitted that all development is meaningless unless some idea of its use, purport, or cause is afforded, and when in the study of the phases of an animal's growth we are led to see prospects of tracing its past evolution, the likenesses and analogies of development become forcibly plain and valuable. Primarily, it may be said, a very large part of the reasonableness of evolution depends on its rational interpretations of development. Without development and its lessons,

evolution would be well-nigh unprovable. Conversely, without the idea of evolution, the development of animal or plant is a meaningless piece of natural transmogrification and change.

In so far as the life-histories at which we have just glanced are concerned, the general conclusions to be drawn from their study lie on the surface of the subject. Beginning with the worms and their transformations, we find a type of larva, presenting a rounded body with variously disposed cilia (Fig. 22), which simply becomes segmented, and with little further change becomes the worm. From the worms to the "Lampshells" is an easy transition, for in the development of the latter (Fig. 19) we find the clearest reproduction of the features of the young worm larva (Fig. 22) in the body divided into its three segments and exhibiting its cilia and eye-spots; whilst, as Huxley remarks, the resemblance to the worm-larva is increased when we find the young lampshell developing bundles of bristles (Fig. 19, F, G), such as the worm possesses, on the middle joint of its body. From such resemblances, Huxley is more than justified in remarking that, whilst the lampshells bear a likeness in development to the plant-like "Sea-mats" and their neighbours (Fig. 19, I), their development "no less strongly testifies to their close relations with the worms." Thus the evolution of a race of lower shellfish from a worm-stock is plainly enough taught by development; and such a fact testifies directly enough to the possibilities of other molluscan developments having had a similar origin.

Coming next in order to the molluscs themselves, we find two classes the bivalves and the gasteropods-in each of which certain primitive forms of development may be traced. The "Veligerstage" (Fig. 7, A) may be regarded as common to both groups; and the common origin for both classes may reasonably enough be argued for and maintained on this ground alone, and apart from any plain agreement in structure. It is, however, in the lowest members of each group that we may expect to find the most marked likeness to the primitive type and root-stock from which these classes have been derived. Hence, it is to Chiton (Fig. 4) and Dentalium (Fig. 9) that we turn for aid in solving the problem before us. The young "Tooth-shell" (Fig. 10, C) is unmistakeably a worm. Its barrel-shaped body, its circlets of cilia, its end tuft of these appendages-all are characters which reproduce before us the embryo worm (Fig. 22, B). Nor is the early history of chiton materially different from that of the "Tooth-shell." The young chiton (Fig. 11, A) leaves the egg, as we have seen, with a ciliated girdle in the middle of its body, and a long tuft of cilia on its head; whilst this embryo seems to

But

proceed even further on the worm-track, when we find its body to become segmented or divided as in the worms (B, C); these divisions becoming the shell-plates of the mature Chiton. Thus Chiton may be regarded, without exaggeration, as a worm-form existing under a molluscan guise. And when we arrive at the higher gasteropods, with their "veliger-stage" and "trochosphere," we see produced before us simply a later modification of the worm stock. The life history of a sea-butterfly or pteropod, in fact, takes up the narrative where the development of chiton ends it. Chiton led us to the worm-larva stage, and thereafter branches off on its lower molluscan path. the pteropod may, as we have seen, begin life as the worm (Fig. 18), and proceeds not merely to develop its "veliger-stage," but remains permanently therein; flapping its way over the surface of the sea by means of the permanent "velum," or its substitute in the form of the fins or wings. Last of all, a gasteropod like Æolis presents us with a multum in parvo of the whole process of gasteropod and molluscan evolution. Here, we take up the story at the stage where the pteropod history concluded. olis and its neighbours, or Lymneus, passing through the pteropod stage, each with its "velum," develop onwards to become the higher and shelled gasteropod, and represent the furthest evolution of the race. Thus, from the worms

to the lampshells on one hand, and to the chitons and "Toothshells" on the other; from these latter, in turn, to the pteropod and thence to the bivalves and gasteropods, the track of development seems plainly marked. The whole story it tells is that of a shifting panorama of the modification of the animal form; phase succeeding phase, and each new succession of forms obscuring, or it may be intensifying, the development of the preceding classes and groups. But, clearly marked or obscure, understood fully or only suggested to the mind, the whole process of development reveals to us the operation of a great law of evolution and progressive change, manifested through those wondrous cycles and transformations which nature seems never weary of exhibiting to the earnest mind and seeking eye.

If, finally, one might be tempted to inquire into the origin of these ciliated worm-larvæ themselves, we may find that speculative natural history is not unprepared with a reply. We are reminded that, as the early changes of egg-segmentation are not peculiar to the molluscs, so neither are the veliger stages the special belongings of that group of animals. The "velum," or its representative ciliated girdles, appears before us equally in the development of the echinoderms or starfish group, of the worm, of the wheel-animalcule or rotifer, and of the mollusc. The zoologist would further remind us that

these ciliary bands often remain in adult animals, and are represented by certain stable possessions, such as tentacles or feelers, gills, the "arms" of lampshells, and like structures. "It is probable enough," say's Professor Ray Lankester, "that all the ciliated bands of invertebrate embryos, even of adult organisms, can be explained as derivatives of one primitive organ." If this thought be fully worked out, it contains a veritable" philosopher's stone" for the zoologist, inasmuch as it enables us to account for the forms and structures of animals on a rational basis. That is to say, the particular form and structure of an animal or class are due to the fashion in which the original ciliated bands of the larva and the embryo itself have been modified by the external and internal forces which now, as of old, operate on living things. Professor Lankester has suggestively worked out this idea of the derivation of all existing embryos from a type-form, to which he has given the name of "Architroch "-a form represented by deputy, so to speak, in certain worms in the sea-mat class, as adult organisms. Such a theory explains to us, on a basis of a reasonable nature, how different forms may arise from a similar root-larva. And it may be added, that should any objection be urged to such views on the

ground that they are entirely hypothetical, one may retort that to all other explanations of the past of nature, whether theological or scientific, exactly similar opposition may be offered. Further, we must reflect, that in any case we have to choose between filling up from our observation of nature the gaps in our knowledge which a philosophical necessity entails, or al

lowing these gaps to yawn unsatisfactorily

The rational mind is not likely to

and permanently unfilled.

hesitate in its choice of alternatives. And if, lastly, it be borne in mind that, so far from being merely shadowy theories, such

ideas of the origin of animal forms are based on close observation of nature-often the work of many concentrated lifetimes-the logical standing of a theory which connects the facts of nature, and by so connecting explains them, needs no justification, as it fears no honest and unbiassed criticism.

(To be concluded.)

ANDREW WILSON.