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with a nucleus, or a darker spot, in the interior. This latter, as Wilson says, may be regarded as "a con* trolling centre of cell activity". It plays a most im- portant part in reproduction, and is probably a con- stituent part of all normal cells, though this point is not yet strictly proved. The amoeba exhibits irrita- bility or movement in response to stimulation. It spreads itself around small particles of food, dissolves them, and absorbs the nutritive elements by a process of intussusception, and distributes the new material throughout its substance as a whole, to make good the loss which it is constantly undergoing by decompo- sition. The operation of nutrition is an essentially inmianent activity, and it 13 part of the metabolism, or waste and repair, which is characteristic of living organisms. The material thus assimilated into the living organism is raised to a condition of chemically imstablc equilibrium, and sustained in this state w^hile it remains part of the living being. When the assimi- lation exceeds disintegration the animal grows. From time to time certain cnaiigcs take place m the nucleus and body of the cell, which divides into two, part of the nucleus, reconstituted into a new nucleus, remain- ing with one section of the cell, and part with the other. The separated parts then complete their de- velopment, and grow up into two dist inct cells like the original parent cell. Here we have the phenomenon of reproQuction. Finally, the cell may be destroyed by physical or chemical action, when all these vital activities cease. To sum up the account of life in its simplest form, in the words of Professor Windle: — "The amoeba moves, it responds to stimuli, it breatlies and it feeds, it carries on complicated chemical pro- cesses in its interior. It increases and multiplies and it may die." (What is Life?, p. 30.)

B. Philosophy. — These various phenomena consti- tuting the cycle of life cannot, according to the School- men, t)e rationally conceived as the outcome of any collection of material particles. They are inexplica- ble by mere complexity of machinery, or as a result- ant of the physical and chemical propert ies of matter. They establish, it is maintained, the existence of an intrinsic agency, energy, or power, which unifies the multiplicity of material parts, guides the several vital processes, dominates in some manner the physical and chemical operations, controls the tendency of the con- stituents of living substance to decompose and pass into conditions of more stable equilibrium, and regu- lates and directs the whole series of changes involved in the growth and the building-up of the living being after the plan of its specific type. This agency is the vital -principle; and according to the Scholastic phi- losopners it is best conceived as the substantial form of the body. In the Peripatetic theory, the /orm or cn- telechy gives unity to the living being, determines its essential nature, and is the ultimate source of its spe- cific activities. The evidence for this doctrine can be stated only in the briefest outline.

(1) Argument from physiological unity. — ^The phys- iological unity and regulative power of the organism as a whole necessitate the admission of an internal, formal, constituent principle as the source of vital activity. Tlie living Iwing — protozoon or vertebrate, notwithstanding its differentiation of material parts and manifoldness of structure, is truly 07ie, It exer- cises immanent activity. Its organs for digestion, se- cretion, respiration, sensation, etc., are organs of one being. They function not for their own sakes but for the service of the whole. The well-being or ill-being of each part is bound up in intimate sympathy with every otner. Amid wide variations of surroundings the living organism exhibits remarkable skill in selecting suitable nutriment; it regulates its temperature and the rate of combustion uniformly within very narrow limits; it similarly controls respiration and circula- tion; the composition of the blood is also kept un- changed with remarkable exactness throughout the

species. In faot, life selects, abeorbe, distributes, stores various materials of its environment for the good of the whole organism, and rejects waste prod- ucts, spending its energy with wonderful wisdom. This would not be possible were the living being merely an aggr^ate of atoms or particles of matter in local contact. Each wheel of a watch or engine — nay each part of a wheel — is a being quite dist met from, and in its existence intrinsically independent of every other. No spoke or rivet sickens or thrives in sympa- thy with a bar in another part of the machine, nor does it contribute out of its actual or potential substance to make good the disintegration of other parts. The combination is artificial; the imion accidental, not natural. All the actions between the parts are tran- sient, not immanent. The phenomena of life thus establish the reahty of a imifying and regulating prin- ciple, energy, or force, intimately present to every por- tion of the Jiving creature, making its manifold iparts one substantial nature and regulating its activities.

(2) Morpho-genetic argument: Growth. — ^The tiny fertilized ovum placed in a suitable medium grows rap- idly by division and multiplication, and builds up an infinitely complex structure, after the type of the species to which it belongs. But for this something more than the chemical and physical properties of the material elements engaged is required. There must be from the beginning some intrinsic formative power in the germ to direct the course of the vast series of changes involved. Machines may, when once set up, be constructed to perform very ingenious operations. But no machine constructs itself; still less can it en- dow a part of its structure with the power of building itself up into a similar machine. The establishment of the aoctrine of epigenesis has obviously increased indefinitely the hopelessness of a mechanical explana- tion. When it is said that life is due to the organisa- tion of matter, the c^uestion at once arises: W^hat is the cause of the organization? What but the formative power — the vital principle of the germ cell? Again, the growing organism has been compared to the build- ing up of the cr>'stal. But the two are totally different. The crystal grows by mere i^gregation of external surface layers which do not afl^ct the interior. The organism grows by intussusception, the absorption of nutriment and the distribution of it throughout its own substance. A cr^'stal liberates energy in its for- mation and growth. A living body accumulates poten- tial energy in its growth. A piece of crystal too is not a unity. A part of a crystal is still a crj'stal. Not so, a part of a cow. A still more marvellous characteristic of life is the faculty of restoring damaged parts. If any part is wounded, the whole organism exhibits its sympathy; the nonnal course of nutrition is altered, the \ital energy economizes its supplies elsewhere uia concentrates its resources in healing the injured part. This indeed is only a particular exercise of the faculty of adaptation and of circumventing obstacles that interfere with normal activity, which marks the flexi- bility of the universal working of life, as contrasted " with the rigidity of the machine and the immutability of physical and chemical modes of action.

The argument in favour of a vital principle from growth has been recently reinforced in a new way by the introduction of experiment into embryology. Roux, Driesch, Wilson, and others, have shown that in the case of the sea-urchin, amphioxus, and other animals, if the embryo in its earliest stages, when con- sisting of two cells, four cells, and in some cases of eight cells, be carefully divided up into the separate single cells, each of these may develop into a complete animal, though of proportionately smaller sise. That is, the fertilized ovum which was naturally destined to become one normal animal, though prevented by arU* ficial interference from achieving that end, has yet at- taimnl its purpose by producing several smaller ani- mals; and m doing so has employed the cells which ft