After defining life, in the third chapter (beginning vol. ii.) he treats of the exciting cause of organic movements. This exciting cause is foreign to the body which it vivifies, and does not perish, like the latter. "This cause resides in invisible, subtile, expansive, ever-active fluids which penetrate or are incessantly developed in the bodies which they animate." These subtile fluids we should in these days regard as the physico-chemical agents, such as heat, light, electricity. What he says in the next two chapters as to the "orgasme" and irritability excited by the before-mentioned exciting cause may be regarded as a crude foreshadowing of the primary properties of protoplasm, now regarded as the physical basis of life--_i.e._, contractility, irritability, and metabolism. In Chapter VI. Lamarck discusses direct or spontaneous generation in the same way as in 1802. In the following paragraph we have foreshadowed the characteristic qualities of the primeval protoplasmic matter fitted to receive the first traces of organization and life: "Every mass of substance homogeneous in appearance, of a gelatinous or mucilaginous consistence, whose parts, coherent among themselves, will be in the state nearest fluidity, but will have only a consistence sufficient to constitute containing parts, will be the body most fitted to receive the first traces of organization and life." In the third part of the _Philosophie zoologique_ Lamarck considers the physical causes of feeling--_i.e._, those which form the productive force of actions, and those giving rise to intelligent acts. After describing the nervous system and its functions, he discusses the nervous fluid. His physiological views are based on those of Richerand's _Physiologie_, which he at times quotes. Lamarck's thoughts on the nature of the nervous fluid (_Recherches sur le fluide nerveux_) are curious and illustrative of the gropings after the truth of his age. He claims that the supposed nervous fluid has much analogy to the electric, that it is the _feu éthéré_ "animalized by the circumstances under which it occurs." In his _Recherches sur l'organisation des corps vivans_ (1802) he states that, as the result of changes continually undergone by the principal fluids of an animal, there is continually set free in a state of _feu fixé_ a special fluid, which at the instant of its disengagement occurs in the expansive state of the caloric, then becomes gradually rarefied, and insensibly arrives at the state of an extremely subtile fluid which then passes along the smallest nervous ramifications in the substance of the nerve, which is a very good conductor for it. On its side the brain sends back the subtile fluid in question along the nerves to the different organs.