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.

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

FIG. 170.-THE ROSY FEATHER-STAR'S DEVELOPMENT.
a, adult starfish; b, young stalked forms.

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. 170) 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.

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The insect world teems with examples of "metamorphosis " at once striking and interesting. It also, however, illustrates a previous remark, that in one and the same class we may find great variations in development and "metamorphosis." For instance, we may find no metamorphosis at all in some insects. The lice, the bird-lice, and the spring-tails (Thysanura) 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 manyinsects of higher rank. A kind of day-fly (Chloëon, Fig. 171) is described by Sir John Lubbock as undergoing no fewer than twenty moultings of its skin during its "metamorphosis," which is not, however, of marked

FIG. 171. CHLOEON.
A, larva; B, perfect insect.

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. 176) 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. 172), 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. Some

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times within the larval skin, or it may be (as in butterflies and moths) 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 change in external form (from the chrysalis to the perfect insect) is

very deceptive, consisting merely of a throwing-off of the outer skin -the drawing aside, as it were, of a curtain, and the revelation of a form which, far from being new, has been in preparation for days, sometimes even for months."

In the metamorphosis of certain of the flies-e.g. the flesh-fliesthe changes are in reality much more sweeping than in the butterflies, although perhaps less apparent than in these brilliant members of the class. The body of the maggot or larval fly contains, when it leaves the egg, a number of curious rounded structures named imaginal discs. Some twelve of these are placed in the young insect's chest-region-four in each segment-and two are

situated in the front part of the maggot's body. No change is perceptible in these discs during the caterpillar or larval stage of the fly's life; but when the maggot encloses itself within the last of its skins, which serves it as a chrysalis case or cocoon, the discs begin to undergo a marked development. Each of the lower discs placed in the insect's chest, develops a leg and half of the segment of the body bearing the leg. The upper discs of the joint give origin to the upper halves of the segment and to the wings. or their representatives; and the two foremost discs are responsible for the development of the head and mouth parts of the perfect fly. As development proceeds,

we find a complete physiological break-down of the chest and head organs of the maggot to be represented. A literally new creature (as to chest and head) is produced and built up from the imaginal discs; the tail or abdomen of the fly consisting, however, of a mere extension and growth of that of the maggot. There is thus witnessed in

FIG. 173.-COCKROACHES,

the development of the fly a much more complete series of changes than in the metamorphosis of a butterfly, and this notwithstanding the fact that the changes undergone by the latter appear to be of more sweeping character than those exhibited by the former insect. Let us bear in mind also the fact, already noted, that the developmental changes in insects may be reduced to a minimum, in respect that many lower insects do not undergo any metamorphosis at all. Even in the cockroach (Fig. 173)-belonging to the Orthoptera, or grasshopper and locust group-an insect familiar enough to warrant its special mention in the present instance, the young possess eyes, feelers, jaws, and legs before they are hatched. They further leave the egg as minute but active insects, whose organs are really moulded upon the type of those which the perfect cockroach possesses. The young insect then undergoes seven moultings or changes of skin. Its first moult occurs when it leaves the egg, and its second takes place about a month afterwards. This second moult being performed, no further shedding of skin takes place until a year afterwards; and as but an annual moult subsequently occurs, the insect does not attain maturity till its fifth summer. Thus, in the cockroach, "metamor

(The left-hand figure represents the male: the female, with rudimentary wings, is represented on the right.)

phosis," in the sense in which that term is used as regards the butterfly, does not occur at all. The male insect, it is true, develops wings at a late stage of development, but there is no chrysalis-stage and no quiescence, as in the butterfly or beetle.

How, then, it may be asked, are the differences between the metamorphoses of insects to be accounted for? On the theory that the development of the individual recapitulates the evolution of the race, it may be asked, do not the facts and differences of metamorphosis in insects seem to present very grave difficulties? There is not that general likeness seen, for instance, in the young of different Crustaceans, nor the similarity in development witnessed in the Echinoderms or starfish group. These apparent difficulties, however, become greatly lessened, or may disappear entirely, if we bear in mind the fact, already insisted on, that as adult animals vary and alter, and so evolve new species, so the young and developing forms are even more subject to modification whilst in the process of growth. In other words, let us clearly understand that the changes an animal or plant may undergo are two in number. Firstly, there are developmental changes, or those we have been tracing in previous papers, which belong to the animal as part of its inheritance, and which cause it to travel along the line of its ancestry towards the likeness of its parent. Then there are, secondly, changes which must be named adaptive; which arise from the operation of surrounding circumstances heat, cold, food, &c.—and from the action upon the living being of external forces. These latter are changes "adapting" it to, it may be, new ways and walks of life, differing from those of its parents and ancestors, and remodelling its frame in a novel guise. The young insect, in truth, may be described as living between two sets of forces or conditions. One set may be named "centripetal," or centre-seeking, for want of a more descriptive term. These are developmental changes which tie it to its type and which cause it to travel along the beaten track of its race. Then there are the "centrifugal" or adaptive forces, which tend to make its development erratic, which may cause it to fly off at a tangent, so to speak, from its normal way of growth, and which necessarily cause it to differ materially from its type and race. Says Darwin, in speaking of development at large: "Many insects, and especially certain crustaceans, show us what wonderful changes of structure can be effected during development." A little later he proceeds to remark that, whilst similar organs in the young of different animals "often have no direct relation to their conditions of existence" (eg. the gill arches of a quadruped, a bird, a frog, and a fish), the case is "different when an animal, during any part of its embryonic career, is active and has to provide for itself. The period of activity," says Darwin, "may come on earlier or later in life; but whenever it comes on, the adaptation of the larva to its conditions