If I should say that only organisms are irritable, I fear that an opponent might present me with many instances in which inanimate objects react to forces that impinge upon them. Should I handle a stick of dynamite with inconsiderate roughness, the response might be more violent than I should desire. A piano responds when I press the keys, and a piece of ice melts in response to the application of heat. Perhaps there is nothing in these reactions of inanimate objects that is essentially different from response in the living organism. A Hindu physiologist, J. C. Bose, has written an interesting book entitled Response in the Living and Non-Living, in which he shows that even metals respond to stimuli by showing fatigue and recovery, and by exhibiting other features of behavior that closely parallel the reactions of living matter.

I come finally to a peculiarity which has often been held up as the very essence of life-the power of adaptation. In an organism, as a rule, activity is directed into useful channels, and the organs by which activities are carried on seem to be especially fitted for their work. Our hands are wonderfully adapted for grasping and manipulating objects, our ears for hearing sounds, our lungs for carrying on the function of respiration, and our hearts for propelling blood. In the plant the leaves, stems, roots, flowers, and fruit are similarly fitted for carrying on their respective functions. Everywhere in the organic world we meet with an adjustment of organ to function. Everywhere nature teems with adaptive contrivances which secure the welfare of her living creatures. The hinge on the bivalve's shell, the protective spines of the sea-urchin, the downy pappus that enables the seeds of the dandelion to be scattered by the wind, the hooks by which the tapeworm keeps its hold on the intestinal wall of its host, are a few among the millions of adaptive devices which the organic world presents.

And it is not only in structural mechanisms that adaptations are shown; they occur even more plentifully in physiological activities and in behavior. The presence of food in the stomach is responded to by setting into action the processes of digestion;

injurious stimuli evoke movements of avoidance; the sight of prey arouses the activities of pursuit in the hungry lion. These actions are adaptive in the sense that they assist in maintaining the life of the organism. Other acts may be serviceable in perpetuating the life of the race. For this end there are specialized mechanisms for reproduction, and instincts for the care of offspring. The mammary glands are adapted by structure and function to supply nutriment to the young, and the latter are adapted to take advantage of this source of food by sucking, swallowing, and digesting. It is obvious that without this highly adaptive instinctive propensity to suck, the whole class of mammals, including ourselves, would speedily become extinct. All living things manifest adaptiveness. These adaptations are not in relation to an external end, like that of a watch for keeping time, for it makes no difference to the watch whether it keeps time badly or well. The end which is served by the organic adaptation is the welfare of the organism concerned. As the wise naturalist-philosopher Aristotle observed over two thousand years ago, life has its end in itself.

Vital activities center about two things, the preservation of the individual organism, and the perpetuation of its race. If we study the various things which organisms do in the course of their life cycle, we shall find that these two employments are what they are engaged in most of the time. In other words, life is bent upon its own continuation. The plant, in absorbing water and nutrient materials from the soil, in developing new buds and shoots, in turning its leaves so as to catch and utilize the rays of the sun, in exhibiting the beautiful color of its petals, and exhaling the fragrance of its nectaries, is simply acting, in one way or another, so as to preserve and enhance its own life, and to provide, through maturing fertile seed, for producing other plants like itself. The life of the lion in seeking prey, avoiding danger, mating, bearing offspring, suckling and protecting them, and later bringing food to the lair to satisfy their lusty growing appetites the whole round of its varied activities-is dominated by the same fundamental endeavor to preserve and perpetuate

its kind. So it is with life everywhere. All organisms are engaged in playing essentially the same game, the game that is as old as life, and that will be played as long as life continues on the earth. It is the game of getting on-succeeding in self-perpetuation and in making provision for the offspring.

If we recur now to the definitions of life to which we previously alluded, and examine some of them which have been proposed, we shall find that they commonly refer to this adaptiveness of life as its chief distinguishing characteristic. Schelling defined life as "individuation"—not a very lucid definition at the first encounter, but a little reflection upon it shows that it emphasizes those self- and race-perpetuating features in which organic adaptiveness consists. G. H. Lewes defined life as "a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity," thus emphasizing, as did Schelling, the element of self-preservation. Herbert Spencer defined life as "the continuous adjustment of internal relations to external relations." Organisms are continually changing within to meet or adjust themselves to changes occurring without. But what is adjustment? Is it not a change which enables the organism to maintain itself, or in other words an adaptive change? Life then is a process of continuous adaptation. Or again, take the definition of Bichat: "Life is the totality of functions that resist death." While this statement has been criticized because it defines life in relation to death, a term which is intelligible only when contrasted with life, the definition really makes self-preservation and race-perpetuation the essential marks of vital activity.

Is this tendency to maintain individuality by a series of adaptive changes a unique possession of living beings? Many objects resist destruction because they are hard or tough. A crystal goes further and ekes out its mutilated form by adding material to its broken surface. A candle flame will quickly regain its regular form after distortion. These are, in a sense, self-preservative reactions, however different they may be from those exhibited by a living organism. Perhaps it is possible to

specify ways in which the adaptive reactions of living beings differ from those of inanimate objects, but even if no absolute distinction can be made, the various peculiarities of life which we have considered constitute a group of characteristics which probably does not occur in its entirety in anything but a living organism. Perhaps we should not be too sure of the truth of this statement, however, for we must bear in mind that life, although apparently a very distinct type of behavior, is carried on in the most intimate relationship to the inorganic world upon which it depends. Possibly all the component processes which are involved in living may occur also in non-living material. Life then would result from their combination in a harmonious and permanent relation.

That living beings derive their material directly or indirectly. from the inanimate world goes without saying. It is also true that the energy for carrying on vital processes comes from outside sources, and for the most part directly or indirectly from the radiant energy of the sun. One of the most important scientific discoveries of the nineteenth century was the law of the conservation of energy, which was established through the labors of Mayer, Joule, Helmholtz, and Grove. According to this law, the energy of any closed system is constant in quantity, although the various forms of energy may be transformed into each other, as when friction produces heat, which in turn may give rise to light, sound, electrical energy, or mechanical work. Organisms, so far as careful experiment and measurement can determine, perform their functions in strict accordance with this fundamental principle. They are vortices in relation to the energy which passes through them quite as much as they are with respect to their component materials. The energy which I expend in writing these words is yielded chiefly by the combustion of carbohydrates in my muscles. These carbohydrates are the product of the constructive metabolism of plant life, the energy of the sun being utilized to build up these compounds out of the simpler constituents absorbed from the soil and air. I simply transform and direct the stream of energy that flows

through my body, without being able either to add to or to subtract from the total amount.

There are many different opinions as to how life is related to the processes of inanimate nature. As science has advanced, more and more of the phenomena of nature have been found to take place in accordance with uniform laws. The changes going on in inanimate things are commonly conceded to occur in conformity with the laws of physics and chemistry. Even the weather, fitful and unpredictable as it may be, we consider as controlled by uniform physical and chemical processes, a complete knowledge of which would enable us to forecast changes with a high degree of accuracy. While weather forecasting is a notoriously unsafe kind of prophecy, the scientist attributes this circumstance to the inadequacy of his knowledge of all the elements involved in his problem, and not to any failure of nature to conform to perfectly definite and invariable rules of procedure. But how about life? Can it be similarly explained in physical and chemical terms? This is the question at issue between the mechanists who hold that life can be so explained, at least theoretically, and the vitalists who contend that it cannot. There are many brands of vitalism, but according to most of them, life activities are somehow controlled and directed by purpose. Life is a kind of purposive striving, bent on accomplishing its own perpetuation. It is fundamentally a teleological, or end-seeking process. Mere physics and chemistry, according to the vitalists, are not adequate to account for the purposive character of living. Physical and chemical changes, they admit, take place in living matter, but these changes are subject to a directive agency, which makes them carry out its conscious or unconscious purpose, much as the powerful engines that propel an ocean liner are guided by the will of the captain. All of the machinery in the engine room may work in strict accordance with the laws of physics, but whether the vessel goes into port at Hongkong or Manila depends upon the guiding purpose of the man at the helm.

The controversy between mechanism and vitalism has been