This mark is like a trace, still visible in each, of what was in the original tendency of which they represent the elementary directions. The elements of a tendency are not hke objects set beside each other in space and mutually exclusive, but rather like psychic states, each of which, although it be itself to begin with, yet partakes of others, and so virtually includes in itself the whole personality to which it belongs. There is no real manifestation of life, we said, that does not show us, in a rudimentary or latent state, the characters of other manifestations. Conversely, when we meet, on one line of evolution, a recollection, so to speak, of what is developed along other lines, we must conclude that we have before U3 dissociated elements of one and the same original tendency. In this sense, vegetables and animals represent the two great divergent developments of life. Though the plant ia
distinguished from the animal by fixity and insensibility, movement and consciousness sleep in it as recollections which may waken. But, beside these normally sleeping recollections, there are others awake and active, just thos^ namely, whose activity does not obstruct the development of the elementary tendency itself. We may then formulate this law: When a tendency splits up in tlie course of its development, each of the special tendencies which thus arise tries to preserve and develop everything in the primitive tendency thai is not incompatible with the work for which it is specialized. This explains precisely the fact we dwelt on in the preceding chapter, viz., the formation of identical complex mechanisms on independent lines of evolution. Certain deep-seated analogies between the animal and the vegetable have probably no other cause: sexual generation is perhaps only a luxury for the plant, but to the animal it was a necessity, and the plant must have been driven to it by the same impetus which impelled the animal thereto, a primitive, original impetus, anterior to the separation of the two kingdoms. The same may be said of the tendency of the vegetable towards a growing complexity. This tendency is essential to the animal kingdom, ever tormented by the need of more and more extended and effective action. But the vegetable, condemned to fixity and insensibility, exhibits the same tendency only because it received at the outset the same impulsion. Recent experiments show that it varies at random when the period of "mutation" arrives; whereas the animal must have evolved, we believe, in much more definite directions. But we will not dwell further on this original doubling of the modes of life. Let lis come to the evolution of animals, in which we are more particularly interested.
What constitutes animality, we said, is the faculty of uttliziog a releasing mecbanism for the conversion of as much stored-up potential energy as possible into "explosive" actions. In the beginning the explosion is haphazard, and does not choose its direction. Thus the amoeba thrusts out its pseudopodic prolongations in all directions at once. But, as we rise in the animal scale, the form of the body itself is observed to indicate a certain number of very definite directions along which the energy travels. These directions are marked by so many chains of nervous elements. Now, the nervous clement has gradually emerged from the barely differentiat- ed mass of organized tissue. It may, therefore, be sur- mised that in the nervous element, as soon as it appears, and also in its appendages, the faculty of suddenly freeing the gradually stored-up energy is concentrated. No doubt, every li\nng cell expends energy without ceasing, in order to maintain its equilibrium. The vegetable ceil, torpid from the start, is entirely absorbed in this work of main- tenance alone, as if it took for end what must at first have been only a means. But, in the animal, all points to action, that is, to the utilization of energy for movements from place to place. True, every animal cell expends a good deal — ^oftea the whole — of the energy at its disposal in keeping itself alive; but the organism as a whole triea to attract as much energy as possible to those points where the locomotive movements are effected. So that where a nervous system exists, with its complementary sense- organs and motor apparatus, everything should happen as if the rest of the body had, aa its essential function, to prepare for these and pass on to them, at the moment required, that force which they are to hberate by a sort of explosion.
The part played by food amongst the higher animab
is, indeed, extremely complex. In the first place it serves to repair tissues, then it provides the animal with the heat necessary to render it as independent as possible of changes in external temperature. Thus it preserves, supports, and maintains the organism in which the nervous system is set and on which the nervous elements have to Uve. But these nervous elements would have no reason for existence if the organism did not pass to them, and especially to the muscles they control, a certain energy to expend; and it may even be conjectured that there, in the main, is the essential and ultimate destination of food. This does not mean that the greater part of the food is used in this work. A state may have to make enormous expenditure to secure the return of taxes, and the sum which it will have to dispose of, after deducting the cost of collection, will perhaps be very small; that sum is, none the less, the reason for the tax and for all that has been spent to obtain its return. So it is with the energy which the animal demands of its food.
Many facts seem to indicate that the nervous and mus- cular elements stand in this relation towards the rest of the organism. Glance first at the distribution of ali- mentary substances among the different elements of the living body. These substances fall into two classes, one the quaternary or albuminoid, the other the ternary, including the carbohydrates and the fats. The albumi- noids are properly plastic, destined to repair the tissues — although, owing to the carbon they contain, they are capable of providing energy on occasion. But the function of supplying energy has devolved more particularly on the second class of substances: these, being deposited in the cell rather than forming part of its substance, convey to it, in the form of chemical potential, an ex- pansive energy that may be directly converted into dther
122 CREATIVE EVOLUTION [ohapT
movement or heat. In short, the chief function of the albuminoids is to repair the machine, while the function of the other class of substances is to supply power. It is natural that the albuminoids should have no specially allotted destination, since every part of the machine has to be maintained. But not so with the other substances. The carbohydrates are distributed very unequally, and this inequality of distribution seems to us in the highest degree instructive.
Conveyed by the arterial blood in the form of glucose, these substances are deposited, in the form of glycogen, in the different cells forming the tissues. We know that one of the principal functions of the liver is to maintain at a constant level the quantity of glucose held by the blood, by means of the reserves of glycogen secreted by the hepatic cells. Now, in this circulation of glucose and accumulation of glycogenj it is easy to see that the effect is as if the whole effort of the organism were directed towards providing with potential energy the elements of both the muscular and the nervous tissues. The organ- ism proceeds differently in the two cases, but it arrives at the same result. In the first case, it provides the muscle- cell with a large reserve deposited in advance: the quantity of glycogen contained in the muscles is, indeed, enormous in comparison with what is found in the other tissues. In the nervous tissue, on the contrary, the reserve is small {the nervous elements, whose function is merely to liberate the potential energy stored in the muscle, never have to furnish much work at one time) ; but the remark- able thing is that this reserve is restored by the blood at the very moment that it is expended, so that the nerve is instantly recharged with potential energy. Muscular tissue and nervous tissue are, therefore, both privileged, the one in that it is stocked with a large reserve of enei^,
the other in that it is always served at the instant it is in need and to the exact extent of its requirements.
More particularly, it is from the sensori-motor system that the call for glycogen, the potential energy, comes, as if the rest of the organism were simply there in order to transmit force to the nervous system and to the muscles which the nerves control. True, when we think of the part played by the nervous system (even tlie sensori- motor system) as regulator of the organic life, it may well be asked whether, in this exchange of good offices between it and the rest of the body, the nervous system is indeed a master that the body serves. But we shall already in- cUne to this hypothesis when we consider, even in the Btatic state only, the distribution of potential energy among the tissues; and we shall be entirely convinced of it when we reflect upon the conditions in which the energy is expended and restored. For suppose the sensori- motor system is a system lilie the others, of the same rank as the others. Borne by the whole of the organism, it will wait until an excess of chemical potential is supplied to it before it performs any work. In other words, it is the production of glycogen which will regulate the consumption by the nerves and muscles. On the con- trary, if the sensori-motor system is the actual master, the duration and extent of its action will be independent, to a certain extent at least, of the reserve of glycogen that it holds, and even of that contained in the whole of the organism. It will perform work, and the other tissues will have to arrange as they can to supply it with potential energy. Now, this is precisely what does take place, as is shown in particular by the experiments of Morat and Du- lourt.' While the glycogenic function of the liver depends Ob the action of the excitoiy nerves which control it, the > Archives lU phytiologie, 1892.
tOHAP.
action of these nerves is subordinated to the action of those which stimulate the locomotor muscles — in thia sense, that the muscles begin by expending without cal- culation, thus consuming glycogen, impoverishing the blood of its glucose, and finally causing the liver, which has had to pour into the impoverished blood some of its reserve of glycogen, to manufacture a fresh supply. From the sensori-motor system, then, everything starts; on that system everything converges; and we may say, without metaphor, that the rest of the organism is at its service.
Consider again what happens in a prolonged fast. It
is a remarkable fact that in animals that have died of
hunger the brain is found to be almost unimpaired, while
the other organs have lost more or less of their weight
and their cells have undergone profound changes.' It
seems as though the rest of the body had sustained th«
nervous system to the last extremity, treating itself
simply as the means of -which the nervous system is tha
end.
To sum up: if we agree, in short, to understand by "the sensori-motor system" the cerebro-spinal nervous system together with the sensorial apparatus in which it is prolonged and the locomotor muscles it controls, we may say that a higher organism is essentially a sensori- motor system installed on systems of digestion, respiration, circulation, secretion, etc., whose function it is to repair, cleanse and protect it, to create an unvarying internal environment for it, and above all to pass it potential
De Hanac^ine, "Quelquei obaervatjona exp^rimen tales sur Tin- sbaolue' ' (Arch. iial. de bichgie, t, xxi., 1894, pp. 322 B.). Recently, analogous obiiervktiona have been made on a man vho died of inanition after b faat of Ibirly-five days. See, on this •ubJMt, In the A nntt biologvpie of 1898, p. 338, the rtisumti of aa article (in Russian) by Tat«kevitch and Stchaaoy.