latter birds, to feed upon flower juices and upon the insects that infest flowers. He emphasises the need for distinguishing clearly between characters or likenesses which are "structural "—that is, are part and parcel of an animal's being—and those that are purely "adaptive"—that is, arise from a similar mode of life, independently of the origin of the species. The former are transmitted from ancestors; the latter are the products of recent modification. The former indicate the true nature of the animal, because they are part of its inheritance; the latter often suggest false resemblances due to similarity of habits and not to community of origin. Thus, whilst the humming-birds and swifts possess structural and inherited likenesses, the former and sun-birds are related only through similar adaptive characters. The skull of a cuttlefish, to select another example, is comparable in its functions to that of a low vertebrate animal, but on no theory of nature are these two groups connected together. They have arisen, like the similarities of sun-birds and humming-birds, entirely independently, in respect probably of similar conditions, and not of inheritance from a common ancestor. The inherited characters which mark real resemblances are not, as we have seen, always apparent; and the adaptive characters through which the life of the species is carried on may entirely mask and conceal them. As Mr. Wallace puts it, we arrive at "the seeming paradox, that the less of direct use is apparent in any peculiarity of structure, the greater is its value in indicating true, though perhaps remote, affinities; while any peculiarity of an organ which seems essential to its possessor's well-being is often of very little value in indicating its affinity for (to) other creatures." Thus we are led to the conclusion, favoured again by development and its lessons, that the humming-birds "are essentially swifts profoundly modified, it is true, for an aerial and flower-haunting existence-but still bearing in many important peculiarities of structure the unmistakable evidences of a common origin."

XII.

THE EVIDENCE FROM THE LIFE-HISTORIES OF INSECTS.

WHEN the development of an animal or plant is duly studied, one or two chief aspects of such a subject fail to be considered by the biologist. Either the young organism has been converted directly into the likeness of its parent, or it has assumed the parental form indirectly and through a series of transformations more or less distinctly marked. In other words, the young form has emerged upon the stage of life in the guise of its parent, or it has appeared first in a shape and under an appearance not recognisable as belonging to the race it has sprung from. In the latter case, changes of greater or less extent convert the young being into the likeness of its progenitors; and when such transformations occur in the life-history of an animal or plant, it is said to undergo "metamorphosis." Every one, for instance, knows that the butterflies of the garden do not emerge from the egg as winged insects, whilst common information is able to assert that they pass through the larval or "grub" stage and also through the chrysalis form before becoming the perfect insects. So, also, the flies begin their life as maggots; and the bees and beetles, with other insects, exhibit like stages to the butterflies in the course of their development. Furthermore, a frog, as we have seen, practically begins life as a fish, breathing first by external and then by internal gills. Sooner or later, however, limbs are developed; the gills are replaced by lungs; the tail disappears; and the tailless condition of the frog race is finally assumed with its emergence upon the land. Insects and frogs-not to speak of other animals, such as crustaceans, whose history has been already discussed in a previous chapter-are therefore said to undergo "metamorphosis."

Sundry questions not unnaturally rise in the mind which attentively considers such phenomena in the animal world. Firstly, there is the plain question, "Why do some animals undergo metamorphosis and others not ?" Then, secondly, may be asked, "What is the meaning of metamorphosis?" or more primarily, "Can any meaning be assigned to this process?" As we have frequently had occasion to point out, such questions receive no aid or solution from that philosophy which maintains, as an article of unquestioning faith, that the living belongings of this world came forth fashioned in all

their excellence-or, it may be added, in all their frequent and apparent imperfections-at the behest of some sudden creative fiat. There is no need to assume development at all on this hypothesis of things, which for the man of science has been slain long ago; though traces of its influence are not unknown in regions removed from the active currents and tides of culture. On the reverse side of matters, stands the theory broadly denominated "evolution," which, seeing the promise of reading a past and progressive history in the developments which pass in panoramic review before our eyes to-day, asserts that a law of progress has guided and still guides life's courses and ways. On this theory we can understand why development takes place-namely, because it is a law of life that the progress and growth of the race should be represented in, and carried out through, its individual histories. And we can also conceivè why development should run in the grooves marked out so conspicuously in many lifehistories, such as those of insects and crustaceans. This latter fact is explicable when it is repeated that we see in an animal's early growth, the lines and stages along which the development of its race has passed. By the very idea of evolution we expect variety and change to be represented in the development of living beings; for such change is the one great condition which has made this universe what it is. Agreeing as to the main reasons for development and its ways, we should find little difficulty in comprehending how these ways and paths have been followed. As we have already impressed upon the reader, the picture is not always clearly limned, and its outlines are often meagre enough. Still, what we do see and know of its form, convinces us of the correctness of the broad deductions of evolution; which deductions being scorned and denied, leave the whole course of nature a tissue of inexplicable absurdities.

In the present instance, dealing with the meanings of metamorphosis, we intend to direct attention to certain details which, for lack of space, have been omitted in previous chapters, and which, dealing with matters of special interest to the student of evolution, may, logically enough, claim attention in a separate section. Such subjects as the general nature of "metamorphosis," and how that process is modified by surroundings and other circumstances, as well as the narration of some life-histories which illustrate very aptly the general conclusions of evolution, may therefore fitly engage our consideration in the course of our developmental studies.

Firstly, then, the general question of "metamorphosis" demands notice. Whilst it is perfectly true that, broadly speaking, only such animals as insects, crustaceans, and frogs-exhibiting very marked and apparent change of form in passing from the young to the adult stage-may be said to undergo "metamorphosis," it would be far more logical, because more true, to assert that the histories of all

living beings, without exception, illustrate the process in question. This remark has been made in reference to the developments we have already studied. For example, there is not, after all, such an immense difference between the development of an insect and that of a fishor, for that matter, between that of the frog and of man himself—when the facts of development are fairly faced and duly understood. No animal or plant is suddenly transformed into the perfect likeness of its parent. On the contrary, it has not merely to grow, but it has to be formed from that which is formless; to become organised by the development of that which has no structure at all; and to advance along lines of development during which it successively assumes a transient likeness to the forms of other and lower beings. Thus a quadruped, whilst undergoing development within its parent's body, in reality passes through as strange and startling a metamorphosis as does a frog outside its parent's body, and external to its egg likewise. A quadruped is really at first like a fish and reptile. So alike are the young of all vertebrates in their early stages, that recognition of the nature of any particular form may be an impossibility. "Metamorphosis" thus occurs in quadrupeds as in frogs; in snails and oysters as in insects. The great and prevailing difference simply exists in the fact that the insect or frog leaves the egg in an imperfectly developed condition and at an early stage of its career, passing the remainder of its development as an independent being. In the quadruped or fish, or in the bird and reptile, the young animal does not quit the parent body or egg at such an early period, but remains within its primitive shelter to undergo its full development- or at any rate to emerge upon the world of active life tolerably well prepared for the struggle of living and being. Even amongst the quadrupeds, as in well-nigh every other group of animals, and as in the plant world likewise, there may be great differences in the degree and stage of perfection at which the young organism is ushered into active or independent existence. No better instance of this could be found than in the case of the kangaroos and their allies, in which, as lower

FIG. 169. YOUNG KANGAROO.

quadrupeds, internal development ceases at a very early period compared with that at which higher quadrupeds are born. The newly born young of a kangaroo, which, when full grown, stands 6 or 7 feet high, measures about one inch in length at birth (Fig. 169), and resembles a little red worm much more nearly than a kangaroo. At birth it is transferred to the characteristic " pouch" of the mother, wherein for weeks it is protected and nourished by the milk secretion. If we consider the effects of growth on such an organism, we may well feel assured that a metamorphosis" of very complete kind must be required to transform the imperfect and feeble being just described,

into the giant quadruped which takes its leap of twenty feet with the utmost ease. So, also, we find in the development of birds wellnigh infinite variety in the stage of perfection at which the young animal is thrown upon its own resources. Of old, naturalists were wont to divide the birds into those which could run about and forage for themselves immediately on leaving the egg, and those which, as mere fledglings, required parental care and attention for a longer or shorter period after bursting the shell. A young chicken is a much more independent being than, say, an infant thrush; and numerous other comparisons might similarly be instituted, with a like result of showing variations in the development of even the animals of a single class.

It seems, therefore, correct to say that the term "metamorphosis" is one of very considerable latitude, and one admitting, in fact, of no rigid definition at all. At the best its value is merely relative, and those animals may be regarded as really most "metamorphic," so to speak, which leave the egg in an immature state, and which, through circumstances which it is our business to trace in this chapter, have to pass through a definite or well-marked set of changes in form, shape, and often of size also, before assuming the likeness of the parental form. If we reflect that every living being springs from a mere speck of protoplasm, devoid of all structure, which we call "germ" or egg," and which contains the potentialities of becoming what its parent now is; or if we further consider that from this speck of albumen there is developed in a few days, as in the case of the chicken, a creature rejoicing in the possession of a complex system of bone, muscle, sinew, brain, nerve, and sense organs-we may well feel inclined to consider such a transformation and development as thorough an example of " metamorphosis" as, and as a far higher development than, that of the insect which attracts our notice simply because it is more evident to our eyes. Another striking proof that "metamorphosis" must be, after all, a comparative term, lies in a knowledge of the fact insisted on and illustrated in a previous chapter-namely, that the eggs of all animals, from sponge to man, pass through the same stages up to and including a given point, at which each group branches off, so to speak, on its own pathway towards adult and specific perfection. Thus, why one animal undergoes those changes of form we see in the insect, and why another does not, are circumstances-to come to details-depending, firstly, on the size of the egg from which it is developed, and concurrently on the amount of nourishment the egg contains; and, secondly, upon the varying circumstances and surroundings of its life, as well as on the life and history of its race, as temporarily represented by its parent. Thus a large-sized egg, with big yolk, will, cæteris paribus, produce an animal in a higher and more perfect stage of