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come to an end by the exhaustion of its forces. The sun must ultimately "run down" like a clock. He thinks that the existing stock of power available for the maintenance of life may last some seventeen million years, but that it must at length be spent. He thus philosophizes, in conclusion, over the phenomena of the final extinction of life:

However this may be, that which most arouses our moral feelings at the thought of a future (though possibly very remote) cessation of all living creation on the earth is, more particularly, the question whether all this life is not an aimless sport, which will ultimately fall a prey to destruction by brute force? Under the light of Darwin's great thought we begin to see that not only pleasure and joy, but also pain, struggle, and death, are the powerful means by which Nature has built up her finer and more perfect forms of life. And we men know more particularly that in our intelligence, our civic order, and our morality, we are living on the inheritance which our forefathers have gained for us, and that which we acquire in the same way will in like manner ennoble the life of our posterity. Thus the individual, who works for the ideal objects of humanity, even if in a modest position and in a limited sphere of activity, may bear without fear the thought that the thread of his own consciousness will one day break. But even men of such free and large order of minds as Lessing and David Strauss could not reconcile themselves to the thought of a final destruction of the living race, and with it of all the fruits of all past generations.

As yet we know of no fact, which can be established by scientific abservation, which would show that the finer and complex forms of vital motion could exist otherwise than in the dense material of organic life; that it can propagate itself as the sound movement of a string can leave its originally narrow and fixed home, and diffuse itself in the air, keeping all the time its pitch, and the most delicate shade of its color tint; and that, when it meets another string attuned to it, starts this again or excites a flame ready to sing to the same tone. The flame even, which, of all processes in animate nature, is the closest type of life, may become extinct, but the heat which it produces continues to exist, indestructible, imperishable, as an invisible motion, now agitating the molecules of ponderable matter, and then radiating into boundless space as the vibration of an ether. Even there it retains the characteristic peculiarities of its origin, and it reveals its history to the inquirer who questions it by the spectroscope. United afresh, these rays may ignite a new flame, and thus, as it were, acquire a new bodily existence.

Just as the flame remains the same in appearance and continues to exist with the same form and structure, although it draws every minute fresh combustible vapor and fresh oxygen from the air, into the vortex of its ascending current; and just as the wave goes on in unaltered form, and is yet being reconstructed every moment from fresh particles of water, so also in the living being, it is not the definite mass of substance, which now constitutes the body, to which the continuance of the individual is attached. For the material of the body, like that of the flame, is subject to continuous and comparatively rapid change—a change the more rapid, the livelier the activity of the organs in question. Some constituents are renewed from day to day, some from month to month, and others only after years. That which, continues to exist as a particular individual is like the flame and the wave—only the form of motion which continually attracts fresh matter into its vortex and expels the old. The observer with a deaf ear only recognizes the vibration of sound as long as it is visible and can be felt, bound up with heavy matter. Are our senses, in reference to life, like the deaf ear in this respect?

The Human Body: An Account of its Structure and Activities, and the Conditions of its Healthy Working. By H. Newell Martin, D. S. C, M. A., M. B., Professor of Biology in the Johns Hopkins University. New York: Henry Holt & Co. 1881. Pp. 655. Price, $2.75.

This work is a contribution to the American "Science Series" of college text-books, and is one of the best of those excellent publications that has yet appeared. Dr. Martin's task in its preparation has not been a light one; for, although he has had the most interesting of all subjects to deal with, and is herein specially fortunate, yet, on the other hand, he has had to compete in the most thoroughly cultivated field of our whole scientific literature. There are many physiological text-books of all grades, and among them are some of the best scientific manuals to be anywhere found. A new work must therefore be of exceptional excellence if it aspires to become a standard on this subject in the higher education.

We have looked over "The Human Body" carefully, and have been interested throughout. The descriptive and explanatory part is remarkably clear, and the accompanying illustrations are abundant and of a superior quality. The book has, moreover, something of a freshness and originality which seemed to be due to the breadth of Dr. Martins preparation as a biologist. One of the difficulties, indeed, with our physiological text-books is, that they have been too generally the work of physiologi-