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X.

THE EVIDENCE FROM DEVELOPMENT (continued). II.—THE LIFE-HISTORIES OF STAR-FISHES AND CRUSTACEANS. ALLUSION has already been made in the preceding chapter to that most fundamental proposition of modern biology which maintains that "community in development reveals community of descent." It has also been shown at length that, in the eyes of modern naturalists, the development of an animal or plant is regarded as affording a clue to the manner of its evolution or descent from pre-existing forms. The formation of a living being to-day, in other words, repeats for us the formation of its race and species in time past. So that, once again to quote Darwin's words, "we can understand how it is that, in the eyes of most naturalists, the structure of the embryo is even more important for classification than that of the adult." Or, again, "embryology (or development) rises greatly in interest when we look at the embryo as a picture, more or less obscured, of the progenitor, either in its adult or larval state, of all the members of the same great class."

Second to none in interest, in the eyes of modern biologists, are the phenomena presented to them in the formation of the animal or the plant frame. In former years the mystery of development was great indeed. There could be offered in the past decade of biology no reason-appealing sufficiently to the rational intellect as explanatory of the events in question-why a frog in its development should appear first as a gill-breathing fish, later on as a tailed newt-like creature, and ultimately as a tailless lung-breathing amphibian. Nor could natural historians in the past. venture to account in more lucid fashion for the curious changes which a butterfly or beetle undergoes in its progress from the days of its youth towards the adult form, and from the stage of the crawling grub, through that of the quiescent chrysalis, to the full-fledged "imago" with its wings. Kirby and Spence summed up and dismissed such matters in a manner-unfortunately for the free play of intellectual vigour, not quite extinct in these latter days-which said much, perhaps, for faith, but little or nothing for reason and science. These famous entomologists held that insects passed through a metamorphosis because "such is the will of the Creator;" and they supplement this "confession of faith" with an attempt at a

scientific explanation by the further assertion that, insects being voracious in their feeding habits, especially in earlier life, perform an important function in the economy of nature in that they remove from the earth's surface "superabundant and decaying animal and vegetable matter." A further reason for this providential arrangement was given in the fact that, as "unusual powers of multiplication" were indispensable for recruiting the ranks of the insect scavengers, and as nutrition and reproduction are incompatible functions, the removal of decaying matter during the youthful stages of the insect's life was to be regarded as a convenient subdivision of its labours, seeing that its adult existence is spent in the work of reproducing its race. But it might easily be shown that, whilst a goodly number of larval insects do feed upon carrion, a large proportion of the class does not exhibit any such habit; and it might reasonably enough be maintained that the argument of Kirby and Spence is open to the serious objection that, in its character, it tends to illustrate the post hoc ergo propter hoc fallacy-Decaying matter exists, therefore insects were designed to pass through a metamorphosis, and were gifted with voracity of disposition that they might remove the said matter from the earth's surface-a proposition vitiated in its exactitude by the fact just mentioned that many insects do not eat such matter; and also by the further facts that many do not undergo a metamorphosis at all; that many voracious caterpillars, instead of eating decaying matter, destroy our trees and flowers. It might also be added that many of nature's scavengers of higher and lower rank than the insects, do not pass through a series of changes in development, but grow, nourish themselves in the exercise of their sanitary work, and likewise, at the same time, and as adult forms, reproduce their species and continue their race in time. Clearly, then, the explanation of Kirby and Spence affords no satisfaction to the contemplative mind in the natural anxiety and desire to discover the causes of things. At its very best, such explanation leaves "the reason why" untouched; and conversely, it can well be understood how any other system of thought, which presents a more satisfactory method of accounting for the facts in question, should find ready acceptance as expanding and enlarging the thoughts of men.

In the previous chapter we discussed the meaning of the remarkable likenesses which can be readily proved, as matters of fact and observation, to exist between the early stages in the development of very different animals. A sponge, a sea-squirt, a lancelet, and even higher animals still, appear in the first beginnings of their existence to pursue a remarkably similar course. Each form parts company with its fellows at a given stage on the way of development, and thereafter passes by the special pathway of its race towards the adult and perfect stage. Von Baer's axiom that development proceeds from the general

to the special, thus declares a great truth of nature. Modern biology appears provided with a host of witnesses to the truth of that axiom, and supplies a reason for the likeness by assuming similarity of descent from lower life as the explanation of those common and general beginnings from which the special and varied forms of animals and plants are evolved every hour around us. The axiom that the development of the individual (ontogenesis) is the rapid shifting or panoramic recapitulation of the development of the species (phylogenesis), is now regarded in biology as the keynote of the whole study of animal and plant formation. If we find, for instance, that the frog in its development is firstly a fish, then a tailed amphibian or newt, and, last of all, a tailless, air-breathing frog, we see in such a panoramic succession of changes-constituting the development of the individual -the evolution and development of the frog race. We read such a history as showing us clearly enough that the frogs have been evolved from some ancient fish-stock, that this fish ancestor became through succeeding modifications a tailed, newt-like amphibian, and finally, that the newt in turn became the higher frog. Most reasonable is the supposition and belief that, if the living hosts have descended from common ancestors, the appearance of ancestral features in their development is a most natural expectation and a highly natural law of life. That transmission from parent to offspring of hereditary features, so familiar to us in human existence-the reproduction of family features by the successive descendants of the family stock-is, in truth, but the repetition in higher life of the likenesses to its ancient ancestry we see in the developing frog. On such grounds, we may attempt successfully to explain the mysteries of development; and on such a principle, we may note in passing, it is easy to see how important a guide to the classification and arrangement of living beings their development affords. If those animals which are descended from a common ancestry resemble each other in their development, such resemblances may be held to represent the truest of those blood relationships which it is the business and aim of classification to express.

The chronicle of the development of animal life is, however, not completed when the earliest changes seen in the formation of the animal frames have been noted. Long after the common and earliest stages, described in the last chapter, have been completed, there may be produced before us marvellous resemblances and likenesses between animals which, when adult, would seem to possess no community either of origin or of other relationship. It is to these later changes in the animal form that we now purpose to direct attention. The history of those changes which more immediately precede the assumption of adult life, affords as valuable evidence of the evolution of species, as does the chronicle of the very beginnings of existence. It is only needful to point out at the commencement of such a study,

that admittedly the panoramic views of evolution we are about to discuss frequently present breaks and gaps in their succession. The expanding canvas of life here and there exhibits a blank surface, due to the part erasure of the picture which, we believe, formerly existed thereon.

There exists a second principle in nature and evolution, of equal importance to heredity or that in virtue of which the likeness of the parent or ancestors is transmitted to the offspring or descendants. This second principle is that of " modification" by adaptation to surrounding or varying conditions. The living being is a plastic unit, capable of being affected and impressed in various, and often undetermined fashions, by the forces of the world in which it lives. Such external conditions-heat and cold, food, habitat, and a host of other circumstances-influence its development in the present, as unquestionably in the past they have modified the history of its race. In truth, the germ-idea of evolution is that of progressive change and alteration induced by the great factors-internal or innate, hereditary and vital forces, and the external or outside circumstances of life. To the operation and influence, then, of surroundings, acting variously upon different natures and organisms, we rightly ascribe the deletion of stages we would naturally expect to meet with in that recapitulation of the animal evolution exhibited in its development. As the geological record, through its imperfections-due to the metamorphism and destruction of fossil-bearing rocks-causes grievous gaps in the history of past life on the earth, so the history and development of the life of to-day shows its blanks and imperfections likewise these blanks caused chiefly, we believe, by the varying outward conditions under which the development of the race was carried out. Thus, if the main outlines of the development of the frog-race be plainly delineated, the pictures likewise may exhibit here but the dimmest possible contour, and may there showa blank. The original fish-ancestor of the race must be sought for amid the fossils-possibly it may never come to light at all. The successive stages whereby the tailed newt became the frog, are barely outlined in the animal world of to-day, and are here and there wanting altogether. But the finger-posts exist nevertheless, and they guide our mental way satisfactorily enough, so long as we trust to their indications. Even though we have to wade through the high tides of difficulty and dimness of knowledge which obscure the intervening ground, we may walk with confidence in that sober path which is founded upon the reason that is attainable.

As Huxley pertinently remarks in a recent manual of zoological instruction : "In practice, however, the reconstruction of the pedigree of a group from the developmental history of its existing members is fraught with difficulties. It is highly probable that the series of developmental stages of the individual organism never pre

sents more than an abbreviated and condensed summary of ancestral conditions; while this summary is often strangely modified by variation and adaptation to conditions; and it must be confessed that, in most cases, we can do little better than guess what is genuine recapitulation of ancestral forms, and what is the effect of comparatively late adaptation. The only perfectly safe foundation for the doctrine of Evolution," continues Huxley, "lies in the historical, or rather archæological, evidence

that particular organisms have arisen by the gradual modification of their predecessors, which is furnished by fossil remains. That evidence is daily increasing in amount and in weight; and it is to be hoped that the comparison of the actual pedigree of these organisms with the phenomena of their development may furnish

some

criterion by which the validity

a

of phylogenetic conclusions (or race-development), deduced from the facts of embryology alone, may be satisfactorily tested."

A survey of some typical groups of animals in relation to their development will provide

us with satisfactory means of judging how far and how plainly the history of the individual repeats that of its race. Turning firstly to some fields of lower life, we may select the class (Echinodermata) represented by the Starfishes (Fig. 107), Sea-urchins (Fig. 106), Sea-lilies (or Crinoids) (Fig. 109), and Sea-cucumbers (Fig. 108), as a starting-point for our inquiries. There is little need that a list of zoological characters should be enumerated by way of impressing the idea of the varied appearance of the animals

2

just mentioned. But it may be remarked that, firstly, they all exhibit