We have emphasized that living matter is continually changing, and this fundamental fact is reflected in nearly all attempts to define life. Aristotle described life as "the assemblage of operations of nutrition, growth, and destruc tion"; deBlainville, as a "twofold internal movement of composition and decomposition"; and Spencer, as "the continuous adjustment of internal relations to external relations." This interaction consists of chemical and physical pro cesses in which combustion or oxidation plays the chief role. Lavoisier and LaplajEgJn 1780 showed that animal heat results from a slow burning of the materials of the body, involv ing the consumption of oxygen and the liberation of carbon dioxide; and further, that for a given consumption of oxygen and liberation of carbon dioxide, about the same amount of heat is produced by an animal as by a burning candle. This was an important discovery, because it went far toward establishing the fact that at least certain characteristic vital phenomena are amenable to the laws which hold in the non living world. But the processes involved in life are not so simple as perhaps might be imagined from the results just mentioned. Heat represents but one of the many energy transformations within the organism. Indeed the living organism, like a steam engine, is a machine for transforming energy trans forming the potential energy stored in chemical complexes of its own substance into the various vital processes of living into work performed. In these processes many complex substances rich in potential energy, which have entered as food and have in whole or part added to the protoplasmic complex, are reduced to simpler and simpler conditions and 16 FOUNDATIONS OF BIOLOGY finally, with their energy content nearly or entirely ex hausted, are eliminated as EXCRETIONS. This continual waste must, if life is to persist, be counterbalanced by a proportionate intake of food in order to renew the supply of energy and afford the materials which, after preliminary changes, are made into an integral part of the living organ ism. Thus in living the animal or plant is partially consum ing and rebuilding itself continually. This dual process is METABOLISM. When constructive metabolism, ANABOLISM, keeps pace with destructive metabolism, KATABOLISM, the individual remains essentially unchanged and this is the normal condition of adult life. During youth the anabolic I phases are in the ascendency and growth occurs, while old I age is characterized by a predominance of katabolic processes. I
3.
Growth
The results of metabolism force themselves upon our attention chiefly as growth, or permanent increase in the size of the individual. As a rule growth in plants con tinues more or less rapidly throughout life, while in animals it is confined mainly to the early part of the individual's existence, or youth. Indeed, at birth a child t i is about a billion times larger than the egg from which it / has developed. Growth means that the organism makes over the materials which it receives in the form of food from its environment and fits them into the protoplasmic organization here and there throughout as needed. This method of addition of materials, which is termed growth by INTUSSUSCEPTION, is highly characteristic of life. When growth occurs in the non-living world, it is typically by accretion; as, for example, in crystals where new material of the same kind is superim posed upon the surface. But protoplasm, with materials THE PHYSICAL BASIS OF LIFE 17 and energy taken from its environment, constructs more protoplasm and, if the available materials are adequate, the specifically organized living substance tends to increase indefinitely. Thus it is not only the method of growth which is diagnostic of animals and plants, but also the fact that when the individual body has reached a certain phys iological balance, or maturity, in which it ceases to increase in size, under normal conditions it expresses the inherent growth power of living matter by setting free certain living units, which go through a cycle of growth phenomena that result in re-productions of the parent individual.
4.
Reproduction
So far as is known, living matter never arises except under the direct influence of preexisting living matter. We have seen that this transformation is continually going on in the constructive phase of metabolism in the animal or plant, and brings about repair and growth of the individual; but it is in reproduction that what may be termed the over; gffiwjj^nf the individual results in the production of a new one. A larger or smaller part of the parent generation is detached and becomes the new generation, so that in ultimate analysis reproduction is division. This is a highly unique characteristic of living things which provides for the con tinuation of the race.
5.
Adaptation
The discussion of metabolism has emphasized the close interrelationship between the living complex and its sur roundings, and the dependence of life upon the interplay and interchange between protoplasm and its environment. As a matter of fact the plant or animal retains its individual ity lives solely by its powers of developing and main 18 FOUNDATIONS OF BIOLOGY taining exquisite adjustments to its surroundings. This results from the IRRITABILITY of living substance : its inherent capacity of reacting to environmental changes by changes in the equilibrium of its matter and energy. The inciting changes, known as STIMULI, may be chemical, electrical, thermal, photic, or mechanical, but the nature of the response is determined rather by the fundamental character of the protoplasmic system itself than by the nature of the stimulus. Muscle protoplasm contracts however it is stimulated. The reaction of living matter by virtue of its intrinsic irritability implies not only response to a stimulus but also conduction so that the protoplasmic system as a whole is directly or indirectly influenced. It responds as a coordinated unit an individual. It adapts itself structurally and functionally to the exigencies of its existence. . This power of adaptation, as exhibited in active adjustment between internal and external relations, overshadows every manifestation of life and contributes, more than any other factor, to the " enor mous gap that separates even the lowest forms of life from the inorganic world."
6.
Organization
Finally, adaptation implies that living things are not homogeneous, but exhibit reciprocal structural and physio logical organization. Accordingly animals and plants are referred to as organisms. Indeed a major part of the present volume is devoted to the organization of organisms. The characteristics which we have described chemical composition, metabolism including waste and repair, growth by intussusception, reproduction, adaptation, and specific organization individually and collectively are diagnostic of living matter. It is possible, to be sure, to take exception to one or another; e.g., to say that growth by intussusception occurs in non-living things when a salt is dissolved in water; but such formal objections only emphasize the unique condi tions which obtain in life. The reader may be surprised to note that the power of movement has not been mentioned as a characteristic of life, but a moment's thought will make it apparent that visible movement is not confined to living matter. Though this is so, movement is one of the most obvious manifestations of life and depends, of course, in every instance, upon molar changes resulting from tumultuous ultramicroscopic chemical changes of protoplasm itself. And it is to these changes that, in the last analysis, we must turn for the energy which brings about the visible movements in animals and plants, such as the contraction of the muscles of animals, the streaming movement (amoeboid movement) of the simple animals known as Amoebae, the rotation and circulation of the protoplasm in certain of the living units of plants and, finally, the lashing of threads of cytoplasm (cilia) which not only enables many a tiny plant and animal to swim, but also aids in numerous ways in certain parts of the bodies of higher organisms. The phenomena of life are quite generally expressed in visible movements, but the latter arc not peculiar to living things. In our discussion thus far we have endeavored to describe the characteristics of matter in the living state on the basis of the fundamental vehicle of life manifestations proto plasm. We have not attempted formally to define 'life' or ' protoplasm ' because they are so unique that it is impossible to resort to the lexicographer's trick of comparing them with something else; and because the expressions 'protoplasm' and 'life' are abstractions; one indicating that all individual animals and plants have to a large extent a common organizational foundation, and the other that they exhibit certain characteristic actions and reactions. The living organism is a microcosm which exhibits a permanence and continuity of individuality correlated with specific behavior, and this it transmits to other matter which it makes a part of itself, and to its offspring in reproduction.
