pass off as carbonic acid gas and water vapor which are no longer parts of the flame.  A fountain is continually replenished by the water which is not-fountain, but which becomes for the time a part of the graceful jet, falling out and away as it leaves the fountain itself.  Just so a living organism is an ever changing, ever renewed, and ever destroyed mass of little particles—­the atoms of the inorganic world which combine and come to life for a time, but which return inevitably to the world of lifeless things.  This is one of the most fundamental facts of biology.  The independence of a living thing like a human being or a crustacean is a product of the imagination.  How can we be independent of the environment when we are interlocked in so many ways with inorganic nature?  Our very substance with its energies has been wrested from the environment; and as we, like all other living things, must replenish our tissues as we wear out in the very act of living, we cannot cease to maintain the closest possible relations with the environment without surrendering our existence in the battle of life.

From the foregoing discussion, it will be evident, I am sure, that there is ample justification for the biological dictum that a living individual is a mechanism.  Not only is the organism composed always of cell units grouped mechanically in tissues and organs and organic systems; not only are the operations which make up its life constant and regular under similar conditions; not only is the whole creature mechanically connected with the inorganic world; but above all the whole activity of a biological individual is concerned necessarily and again mechanically with the acquisition of materials endowed with energy, which materials and energy are mechanically transformed into living matter and its life.  Even though an organism is so much more complex than a locomotive, and so plastic, nevertheless, in so far as both are mechanisms, the conception of the evolution of the former may be much more readily understood through a knowledge of the historical transformation of the latter.

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What, now, is life?  To most people “life seems to be something which enters into a combination of carbon and hydrogen and the other elements, and makes this complex substance, the protoplasm, perform its various activities.”  Nearly every one finds it difficult to regard life and vitality as anything but actuating principles that exist apart from the materials into which they enter, and which they seem to make alive.  According to this general conception, “life is something like an engineer who climbs into the cab of the locomotive and pulls the levers which make it go,” as health might supposedly be regarded as something that does not inhere in well-being, but gets into the body to alter it.  But is this conception really justified by the facts of animal structure and physiology?  Let us recall the steps of our analysis.  The living