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. 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 snail and oyster, on the other hand, 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 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. 1), and resembles a little red worm much more nearly than a kangaroo. At birth it is transferred to

FIG. 1. YOUNG KANGAROO.

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, a 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 terin " 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 paper, 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 paper-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 under

goes 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 a big yolk, will, cæteris paribus, produce an animal in a higher and more perfect stage of development than a small egg, in which no provision exists for the nutrition of the embryo. So much, indeed, may safely be predicted of the causes which retard or favour an early escape from the egg. In the latter case, of course, let us bear in mind that the young will not resemble the parent animal, and we naturally expect to behold changes of form or "metamorphosis" in its further development, and ere it attains to the parent size and likeness.

K

But we must not neglect to note an equally important cause of alteration in form which, acting subsequently to the escape of the immature animal from the egg, will direct its footsteps in different channels, and clothe its form with varied guises. The surroundings of an animal's life necessarily affect that animal, and in time its race, viewing individual and race as consisting each of an adult being and beings. This much is the plainest of plain truths. But it is equally true that surroundings and varying conditions of life must also affect the young stages of animal existence. Even more marked and powerful must be the effect of outward conditions on the young organism, whose frame and constitution, not yet fully formed, are infinitely more plastic and facile than those of the adult. All we know of the effects of environments on living beings teaches us this lesson, We know something of the effects of

heat and cold, of a change of medium, and of numerous other circumstances which materially alter the development of both animals and plants. Natural-history records teem with examples of these facts. A young rosy feather-star (Antedon, Fig. 2) may be hurried through its larval state, and may be made to gallop post-haste through its "metamorphosis," if it be supplied with pure sea water. If, on the other hand, such a larva be kept at a low temperature, and in water not frequently changed, and consequently on a more meagre dietary, it will delay in its larval progress. Its development may not merely be greatly protracted and prolonged, but it will attain to a higher stage of independent development than before. So also with many insect larvæ, and so with zoophytes. The effects of varying conditions on the young and developing animal are plainly traceable. It remains for us to discover what light such reflections throw on some well-marked and familiar cases of metamorphosis around us. The insect world teems with examples of "metamorphosis" at

FIG. 3. CHLOËON.

A, larva; B, perfect insect.

sanura) thus come from the egg resembling in every respect, save in size, the perfect insects. They simply cast or shed their skin at each successive stage of growth, but no change of form is represented in their development. So also with many insects of higher rank. A kind of day-fly (Chloëon, Fig. 3) is described by Sir John Lubbock as undergoing no fewer than twenty moultings of its skin during its "metamorphosis," which is, however, not of marked or distinct character, since the organs of the young animal are simply and gradually changed into those of the adult insect. Even in insects which undergo a much more typical metamorphosis than the day-flies, the gradual conversion of the larval parts into the

organs of the adult may be witnessed.

A young cricket (Fig. 8)

becomes the adult very gradually, and the days of its infancy are not markedly separated from those of its youth, nor are these latter in turn sharply defined from the period of adult life. Turning, however, to actual details, we find a butterfly (Fig. 4), fly, and beetle respectively to exhibit the so-called "perfect" form of metamorphosis. Each begins life that is, comes from the egg, after the preliminary stages common to all eggs-as a grub, caterpillar, or larva (a), which spends the first part of its existence in the guise of a worm, eating voraciously and increasing, as a rule, many times its original size in bulk. Next this voracious grub settles down and becomes the chrysalis or pupa. Here quiescence is the order of the day. Sometimes within the larval skin, or it may be (as in butterflies and moths)

FIG. 4. METAMORPHOSIS OF SWALLOW-TAILED BUTTERFLY.
a, larva; b, chrysalis; c, imago, or perfect insect.

within a special case or cocoon (b), the chrysalis passes its existence, which, however quiet and apparently unimportant, externally viewed, is nevertheless marked by a wonderful activity inside. There the elements and nutrient parts of the larva, accumulated during its season of epicurean enjoyment, may be practically broken down, and rebuilt to form the body of the perfect insect, as in some flies, or more gradually changed into the adult organs, as in the butterflies. As Sir John Lubbock succinctly puts it, "the change from the caterpillar to the chrysalis, and from this to the butterfly, is in reality less rapid than might at first sight be supposed; the internal organs are metamorphosed very gradually, and even the sudden and striking