method of deriving potential energy from plant reservoirs it likewise stores up potential energy for itself by the direct action of the sun's rays on its chlorophyll. But in the course of higher development it was found that these two functions, that of storing up energy and its expenditure in free movements, were incompatible in the same organism. There thus opened out before the organism two lines of development, one of greater movement, but with all the hazard of an uncertain food supply, the other of fixity, but with a certainty of food supply; the former resulted in the animal kingdom, the latter in the vegetable.
Since, however, these two kingdoms are branches of the same life impetus each contains something of the other. The difference lies merely in the tendencies upon which each lays emphasis, while it leaves the other tendencies lying dormant. So that plants and animals can not be defined by mutually exclusive characters, but rather by the accentuation of certain tendencies. Plants take their food as a rule from the inorganic, animals from the organic; as a result plants are usually fastened to the earth, immobile; animals get their food through movement. As a consequence of this differing method of food getting the plant cell surrounds itself with a hard coat of cellulose through which external stimuli can with difficulty affect the organism, and there is hence made possible but a very slight consciousness. Since to the animal cell movement is essential to food getting, it can not completely encase itself in a hard external skeleton; it thus follows that external stimuli readily affect the organism and there is hence rapidly developed an ever higher type of consciousness.
Consciousness, as used by Bergson, is not limited to self-consciousness, but is the kind of consciousness that Jennings in his "Behavior of Lower Organisms" is inclined to believe is possessed by all animals from the highest to the lowest. Bergson relates it to mobility. "The humblest organism is conscious in proportion to its power to move freely."
The elements into which a tendency splits do not possess the same power to evolve. The truly elementary tendencies continue to evolve, leaving behind the residual, split-off tendencies. This is illustrated in the development of the plant kingdom, where it is the carbon-fixers which carry on the main line of evolution.
Along the animal pathway, three of the main branches are those of the mollusks, arthropods and vertebrates. During the middle Paleozoic all had run into the blind alleys of stagnancy, of torpor, since most forms of these phyla had become enclosed in a hard external skeleton; but before this condition had become universal, some of the arthropods assumed, instead of the hard external skeleton of the crustacean, the soft one of the insect, and among the vertebrates the armored fish gave place to the unarmored.
