organic compounds containing nitrogen, which thus seems a fitting element to enter into the composition of anything so prone to change as is living matter.

IV. In every animal and plant these four elements (oxygen, hydrogen, carbon, and nitrogen) unite to form a special substance known as protoplasm, of which every living organism is at first entirely composed, while the whole inorganic world is destitute of such material.

This curious substance, while living, has six very remarkable powers:

(1.) A power of internal circulation, or of the movement of various parts of its substance within the whole, unlike anything in the inorganic world.

(2.) A power of contraction and expansion under conditions different from those which contract and expand inorganic substances.

(3.) A power of performing chemical changes and evolving heat more gently and continuously than in the combustion of inorganic bodies.

(4.) A power of converting other adjacent substances into material like itself-into its own substance.

(5.) A power of forming from its own substance substances both different from its own and from substances adjacent to it. Thus it is that since every living creature consists at first entirely of protoplasm, every other kind of substance found in every animal or plant comes from protoplasm and is formed by its agency.

(6.) A power of exchanging gases with its environment-notably of absorbing oxygen and giving out carbonic acid.

These exclusively vital powers of living particles of protoplasm give to each whole organism of which they form a part certain further characters by which they all differ from the inorganic world. Thus:

V. Every living creature, whether plant or animal, effects that interchange of gases just mentioned (absorbing oxygen and giving out carbonic acid), that is to say, it respires or breathes-whatever other changes it may effect.

VI. Every living being is a creature requiring food, which it has the power of changing into its own substance, and so, at least for a time, augmenting its size by a process of growth. This growth is not a mere external increment, like the growth of a crystal suspended in a suitable medium, but is an augmentation of its intimate innermost substance by what is called intussusception.

VII. Every living creature thus grows according to a more or less definite law, from a single, minute, spheroidal mass of protoplasm into that shape and structure which is characteristic of the group to which it belongs.

VIII. In this process each such creature forms certain substances which are not protoplasm,-at the very least it forms minute granules which may be fatty or starchy; while, generally, living creatures do form the most complex structures, namely, all those found in the animal and vegetable kingdoms-the woods, resins, oils and sugars of plants, and all the varied components of the bodies of animals; this process is known as "secretion."

By this latter process the living world, as one whole, is continually taking matter from the earth's aërial and aqueous envelopes and adding it to the substance of the earth's solid crust. The past effect of this action we see, as before mentioned, in the enormous fields of coal and peat; in the extensive chalk formations and coral reefs (one reef extending for a thousand miles along the coast of Australia, and such structures forming a great part of Florida); in the vast accumulations of fossil remains-evidenced by the fact that the fossil ear bones of whales (a valuable manure) have given rise to a lawsuit, and by the five million cubic feet of shell-sand annually collected on the shores of Devon and Cornwall.

As to the present activity of the vegetable world in this direction, we have but to recollect that the Empire of Brazil is mainly a forest region which may be roughly represented as an equilateral triangle, each side of which is twelve hundred miles long, and that other vast regions of the earth's surface are, like it, clothed not only with herbage, but with teeming vegetable produce of all kinds and dimensions.

Now, if we suppose two-thirds of the earth's dry land to be clothed with only such vegetation as may be estimated to produce an average increase of its substance, amounting to but one three hundred and sixty-fifth part of an inch daily, then we should have freshly formed each year as much vegetal matter as would constitute a cube fifteen miles in extent in each of such cube's three dimensions!

IX. But living creatures not only grow and develop their own. bodies; they also reproduce their kind; and this is again an action to which there is nothing comparable or analogous in the whole inorganic world.

Thus every living being may be said to be a creature possessing an innate tendency to undergo a definite cycle of changes when exposed to certain fixed conditions; that is, when supplied with an adequate amount of temperature, moisture, suitable gaseous matter, food, etc. Inorganic and dead substances may tend to undergo a series of changes, but such series never constitutes a "cycle"-i.e., a series returning to the point whence it set out. We see such a cycle of changes in the egg, the chick, the fowl, and the egg again; or the egg, the grub, the chrysalis, the butterfly, and ultimately its

egg; or the seed, the young plant, the mature plant, the flower, the fruit, and the seed again.

Inorganic substances tend simply to persist as they are, and have no definite relations either to the past or to the future. Whence it comes, or what it has been or shall be, is nothing to its present being which is its only being. But every living creature, at every step of its life, regards both the past and the future, and thus lives continually in a definite relation to both these as well as to the present. Every stage of its cycle of life, just because it is a cycle, is conditioned by the anterior states which alone have made it possible, and refers to future states for which it is in active preparation. Thus, as it were, at every present moment of its existence, it lives both in the past and in the future, a mode of existence which attains its fullest development in the highest living organism— man, the one creature emphatically, because consciously, "looking before and after!"

X. But living creatures present another still more distinctive character, one which is indeed but obscurely indicated in plants, but is very evident in animals. This is the power of forming habits, which is itself the sign of the possession of a special internal spontaneity in living things, by which they each and all tend to act and to "react" when acted upon.

For what is a "habit?" A "habit" is not formed by repeated actions, though it may be strengthened and confirmed by them. If an act performed once only had not in it some power of generating a "habit," then a thousand repetitions of that act would not generate it. Habit is the determination in one definite direction of a previously vague tendency to action. All living organisms tend to act. With them action is not only their nature, 'tis a positive want. Moreover, within limits, the powers and energies of living creatures increase with action, and diminish, and finally perish, through repose. Thus the general activity and power of organisms, and also the exercise of this power in definite modes and directions, are facilitated and increased by actions in the very first of which the power of "generating habit" lies hid.

This second, mysterious, internal tendency, as we have said, eminently distinguishes living organisms from all inorganic bodies, and leads naturally to the next point we would refer to.

Closely allied to habit is instinct, a power, the presence of which cannot indeed be adduced as a character distinguishing all living beings from bodies devoid of life, but which none the less is so remarkable a property of many animals that it may well claim, for our present purpose, to be here briefly referred to in passing.

We have no space here to describe at length examples of animal instinct; we can but very briefly refer to such well-known instances

as the simulated lameness of certain birds, the insects which become quiescent to escape an enemy (what is wrongly called shamming death), and provision for the future, as in the wasp sphex, the carpenter bee and the stag beetle. Certain instincts, however, have a very peculiar significance; such are those by which a grub will repair its injured cocoon or a spider its injured web, and those by which lobsters and crabs, when one of their limbs is injured, will throw off the injured stump as far up as one of its joints, whence alone the limb can again grow forth and be reproduced. Such creatures cannot be supposed to know the effect of such spontaneous amputations, and therefore their actions lead us naturally to consider other unconscious organic actions by which lost parts are more or less perfectly reproduced-actions which display a purpose and intention (although unconscious) in a way which resembles nothing in the inorganic world.

In the process of healing and repair of a wounded part of our body, a fluid, perfectly structureless, substance is secreted, or poured forth from the parts about the wound. In this substance small particles of protoplasm, called “cells," arise and become abundant, so that the substance, at first structureless, becomes what is called "cellular tissue." Then, by degrees, this structure transforms itself into vessels, tendons, nerves, bone, and membrane-into some or all such parts-according to circumstances.

In a case of broken bone its two broken ends soften, their sharp edges thus disappearing. Then a soft substance is secreted, and this becomes at first gelatinous, often afterwards cartilaginous, and, finally, osseous or bony. But not only do these different matters arise and develop themselves in such a neutral substance, but very complex structures, appropriately formed and nicely adjusted for the performance of varied functions, may also be developed. Thus a certain railway guard had his arm so injured that he was compelled to have the elbow, with its joint, cut out; but he afterwards developed a new joint almost as good as the old one. In the uninjured condition of these parts, the outer bone of the lower arm— the radius-ends above in a smooth-surfaced cup, which plays against part of the lower end of the bone of the upper arm, or humerus, while its side also plays against the side of the other bone of the lower arm (called the ulna) with the interposition of a cartilaginous surface. The radius and ulna are united to somewhat descending processes, at the lower end of the humerus, by dense and strong membranes or ligaments. Such was the condition of the parts which were removed by the surgeon. Nine years after the operation the patient died, and the well-known surgeon, Mr. Syme, had the opportunity of dissecting the arm, which in the meantime had served the poor man perfectly well, he having been VOL. XI.-5

in the habit of swinging himself by it from one carriage to another while the train was in motion, quite as easily and securely as with the other arm. On examination Mr. Syme found that the amputated end of the radius had formed fresh polished surfaces and played both against the humerus and ulna as before, a sort of cartilaginous material being freshly interposed. The ends of the bones of the forearm were again locked in by two freshly formed descending processes of the humerus, and were again joined to the latter by freshly formed strong and dense ligaments. Repairs of injuries of a far more surprising kind are found amongst the lower animals, and repair in the vegetal world is so common that it ceases to excite our surprise. Such unconscious and purposive organic actions are allied to instinctive action, using that term in a wide analogical sense. But truly instinctive actions take place IN US at the dawn of life. It is by the aid of such alone that the infant lives. Instinctive also are many of the phenomena of adolescence and those of the earlier years of our own race—for no one can maintain that the first beginnings of literature, art, science, or politics were ever deliberately invented.

How, then, are we to regard that great world of living creatures, both the lower and the higher members of which present phenomena so different from anything to be found in the whole inorganic world? Are, or are not, the bodies of animals and plants vehicles for the exhibition of some force or energy radically different from any to be found in the non-living world about them, or are all their actions to be regarded as only the very curious activities of very complex machines, moved by no other power than such as are inherent in the inanimate matters of this planet? Are we, in a word, to accept a merely mechanical explanation of the universe, or must we demand something more, and if so, what?

To many of our readers it may seem altogether absurd to attempt to explain the phenomena of life in terms of the movements of solid particles. Their common sense revolts at such an explanation, but "common-sense" cannot be allowed by itself to decide any question when an appeal has once been made to the higher tribunal of pure reason, and such an appeal has been made.

For there can be no question but that a thoroughly mechanical conception of nature is the scientific ideal of a very large and a very influential school of thinkers, and is the goal towards which they strive following the footsteps of their great predecessor Descartes. Thus Kirchenhoff tells us that "the highest object at which the natural sciences are constrained to aim, is the reduction of all the phenomena of nature to mechanics." Helmholz has declared that "the aim of the natural sciences is to resolve themselves into mechanics." According to Wundt, "the problem of physi