with hair, claws, or finger-nails, and is only modified skin. When a section through a piece of skin is examined with the aid of a moderately-powerful microscope, the lower or internal surface is seen to be made up of little, irregularly-rounded cells, or bags, with soft semifluid contents; and, while the animal is alive, new cells are constantly forming under the old ones, which are pushed outward and crowded together, and gradually lose their soft contents, and are flattened out into very small scales. The outer layer of the skin is made up of

these scales, which are fastened to each other firmly enough to be separated from the living layer below in a thin sheet, as happens when a blister is raised on the hand by unusual work, but in most parts of the body they are slowly rubbed off as new ones grow; but at the tips of the fingers they are so firmly united that they form horny plates, or "nails," which are pushed forward as new cells form at the root.

In the skin of a bird where a new feather is to grow there is a little pit, and, at the bottom of this, an elevation or pyramid; extending up one side of this pyramid is a groove, or furrow, deepest at the base, and gradually growing shallower until it disappears near the top; from each side of this furrow a great many smaller grooves extend around to the other side of the pyramid, and these also decrease in depth, and at last disappear just as they are about to meet on the side opposite the large furrow. The whole pyramid is covered with skin, and the surface is made of the same scales, or flattened cells, that are found over the rest of the surface of the body; but, instead of falling off when they are pushed out by the new ones below them, they become united or welded to each other, so as to form a horny coat over the surface of the pyramid, with ridges on its lower or inner surface, corresponding to the grooves on the pyramid; and, as new cells grow at the base, this coat or cast of the surface is pushed upward till it breaks at its thinnest part, which is, of course, the smooth part without ridges opposite the large furrow; and then, as it is

pushed outward and flattened, it assumes the form of a feather, the ridge formed in the main furrow being the shaft, while the casts of the side grooves form the separate barbs of the vane. When all of the vane has been formed and pushed forward, the pyramid loses its grooves and becomes smooth, and the wall now formed on its surface, being of the same thickness in all parts, does not break, but remains tubular and forms the quill, which is attached to what is left of the pyramid. A finger-nail or a hair is formed from the same kind of scales in the same way, the process differing only in those features which give to each organ its special character. Feathers, scales, hair, claws, and nails, all are made alike from the dead, flattened cells crowded to the surface by the process of growth.

If, passing from the feather to the wing, we study that in the same way, we shall find that it is made, part for part, on the same plan as the arm of a man, the fore-leg of a horse, the fore-foot of a turtle or frog, and the fin of a fish; and, when these organs are compared in their earlier stages of growth, the resemblance is very perfect; and it is only as one becomes fitted for swimming, another for flying, another for running, and another for handling and feeling, that the differences between them begin to appear. Studying now the whole body of the bird in the same way, and comparing it with a mammal, as the horse; a reptile, as the turtle; a batrachian, as the frog and a fish-we find that all these animals are constructed on the same general plan, and here, also, the resemblance is stronger in the earlier life of the animals. We find, however, that they do not all resemble each other in the same degree, for the bird is more like the turtle than like any of the others, and, when full grown, it preserves some resemblance to reptiles; and there is an animal, found only in the fossil state, called the archiopteryx, which unites in itself many of the characteristics of birds, such as the possession of feathers, with other characteristics as unmistakably reptilian.

Such are the principal facts to be learned about the wing, and any explanation of its origin must account for them all; and the same or similar facts may be learned by studying almost any organ or animal.

To recapitulate: they are, first, the wonderful adaptation of all parts for their uses, rendered still more wonderful by the second fact, that the parts so adapted are modified forms of what are called homologous organs, that is, organs having the same plan, but adapted to quite different uses, and having very little superficial resemblance; third, the fact that, when the growth of these homologous parts is compared, it is found that in their earlier stages they are very much alike, and differ so far as and at the same time that they acquire those characteristics that fit them for their special uses; fourth, is the fact that there are or have been animals whose structure has been so little modified that they seem to connect animals of very different but homologous structure.

Now, what is the meaning of these relations between organs and between animals? For that they have a meaning must be clear to all, and it is fair to presume that it is one that can be discovered by investigation.

The fact that two or many different animals are constructed on the same plan seems to indicate some kind of connection between the animals themselves, and it is the work of the zoologist to find what it is that thus connects them.

Two theories have been proposed, each of which seems to meet most of the points to be explained, but each seems to fail in some respects. One of these is, that the connection between different groups of animals is to be found only in the mind of their Creator; the other is, that there is a direct genetic connection or relationship between them.

Each of these theories is conceivable and worthy of consideration, for we can find examples of the building up of systems somewhat resembling the animal kingdom in each of these ways. The various kinds of steam-engines, for instance, are adapted each to its special work, with an accuracy rivaling that of Nature, yet all of them can be shown to be constructed on substantially the same plan.

If we trace the history of any form, such as the steep-grade locomotive, we find, as we go backward, that it loses, one by one, all of its special adaptations, until at last it is only a common locomotive at up-hill work. Tracing the history of the locomotive in the same way, we find that its special adaptations disappear, until it is nothing

but a power-engine placed on wheels-not the improved power-engine of the present day, but an unimproved and rudimentary form. We might trace the history of other forms in the same way, until we had found the one source for them all.

In this case there can be no genetic connection; each engine is ́an independent thing, and the only connection is an ideal one in the minds of the inventors; that is, the idea of the steam-engine has gone through a process of evolution, expansion, and perfection, and most of the steps in the process have been embodied in real engines, so that together they form a manifestation or record of the changes that the idea has undergone.

According to the theory of which Agassiz is the most celebrated advocate, the phenomena of life are to be explained in a somewhat similar way. Recognizing all the facts which seem to indicate the evolution of the animal kingdom, and being himself the discoverer of very many of them, he says that the evolution is simply the evolution of an idea in the mind of the Creator, which idea has been embodied in material form in such a way that it can be traced by the study of the animals that form its expression.

The other theory may also be illustrated by an example: When we compare languages which philologists tell us have descended from one parent tongue, we are attracted by their differences only, and it needs careful study and comparison to understand the similarity of plan which underlies them all; but when their history is traced it is seen that they were originally the same, and have become different as the races using them have become more widely separated, and, coming under new and widely different physical conditions, have diverged in their habits, feelings, thoughts, and associations, and have required different forms of speech to supply their need. Here, unlike the case of the steam-engines, the language has been the same all the time, and, although men have been the means by which the change has been effected, they have not been the intelligent cause, but have been unconsciously acted upon by agencies around them.

According to the theory with which Darwin is identified, although he is not by any means the author, but has simply removed some of the most serious objections, all the different forms of life have been evolved from one source in substantially the same way that languages have originated; as animals become exposed to new conditions, new varieties adapted to these conditions arise, and, as animals thus grow different, the parent unimproved forms are unable to struggle with their more perfect descendants and become extinct, so that the animals which would connect dissimilar forms are no longer in exist

ence.

The evidence necessary for the perfect establishment of either of these theories does not seem to have been obtained as yet, and we can only decide provisionally, according to probabilities; but the discus

sion of the evidence already collected, or even a bare outline of it, would lead us far beyond the limits of this article, which is simply designed to show how much food for study even a feather will supply, and what broad questions it will lead us into.

WHAT THE CHEMISTRY OF THE ROCKS TEACHES.

IT

By C. C. MERRIMAN.

T is a general rule that substances can crystallize only while solidifying from the liquid state of either fusion or solution. The only exceptions are, that some few substances crystallize directly from their vapors without passing through the intermediate liquid form. Now, the older unstratified rocks of the geological formations, as the granites, are unquestionably fusible, are crystalline in their structure, and are practically insoluble. Therefore the evidence is conclusive that they were all at one time in a molten, fluid state.

Thus far, it would appear, geologists are agreed, since they have named these formations the igneous rocks. But, whether the melted minerals were ever heated to a higher degree than fusion—that is, to the condition of vaporized elements-is an inquiry either carefully avoided by the authorities in geology, or merely mentioned as pertaining to an ingenious hypothesis which, it is claimed, is unsustained by any sufficient proof. It remains to be seen, however, if this theory of the original gaseous form of the material elements does not follow as a necessary consequence from the chemical constitution of the rocks themselves; and if it does not explain and bear testimony in geological and cosmical sciences to such an extent as to make it absolutely essential to them.

The question here presented resolves itself into two alternatives: Either the materials of the earth's crust were formed, according to chemical laws, out of the simple elements preëxisting in liquid or gaseous form, or they were created in the condition of melted and oxidized masses ready to cool into granite and limestone. The latter supposition will hardly be seriously entertained in these days of free inquiry into the natural causes of things. It is now not only conceded, but expected, that science shall have sole jurisdiction in every case where compound bodies are the subject of investigation. To follow them back to the primal laws and elements of their being-to reveal the cause and manner of their birth among the atoms-is now the highest aim of inductive research. On this border-line of inquiry, where the known shades off into the unknown, and the finite into the infinite, science has of late gained its most signal triumphs. And it scarcely requires a prophetic sense to discern that the groundwork of