In the same work (1802) Lamarck defines thought as a physical act taking place in the brain. "This act of thinking gives rise to different displacements of the subtile nervous fluid and to different accumulations of this fluid in the parts of the brain where the ideas have been traced." There result from the flow of the fluid on the conserved impressions of ideas, special movements which portions of this fluid acquire with each impression, which give rise to compounds by their union producing new impressions on the delicate organ which receives them, and which constitute abstract ideas of all kinds, also the different acts of thought. All the acts which constitute thought are the comparisons of ideas, both simple and complex, and the results of these comparisons are judgments. He then discusses the influence of the nervous fluid on the muscles, and also its influence considered as the cause of feeling (_sentiment_). Finally he concludes that _feu fixé_, caloric, the nervous fluid, and the electric fluid "are only one and the same substance occurring in different states." FOOTNOTES: [107] Charles Bonnet (1720-1793), a Swiss naturalist, is famous for his work on Aphides and their parthenogenetic generation, on the mode of reproduction in the Polyzoa, and on the respiration of insects. After the age of thirty-four, when his eyesight became impaired, he began his premature speculations, which did not add to his reputation. Judging, however, by an extract from his writings by D'Archiac (_Introduction à l'Étude de la Paléontologie stratigraphique_, ii., p. 49), he had sound ideas on the theory of descent, claiming that "la diversité et la multitude des conjunctions, peut-être même la diversité des climats et des nourritures, ont donné naissance à de nouvelles espèces ou à des individus intermédiaires" (_Oeuvres d'Hist. nat. et de Philosophie_, in-8vo, p. 230, 1779). [108] See his remark: "_On a dit avec raison que tout ce qui a vie provient d'un auf_" (_Mémoires de Physique_, etc., 1797, p. 272). He appears, however, to have made the simplest organisms exceptions to this doctrine. [109] _Elementa physiologiae corporis humani_, iv. Lausanne, 1762. [110] _Theoria generationis_, 1774. [111] _Mémoires de Physique_, (1797), p. 250. [112] _Mémoires de Physique_, etc. (1797), p. 272. [113] Huxley's "Evolution in Biology" (_Darwiniana_, p. 192), where be quotes from Bonnet's statements, which "bear no small resemblance to what is understood by evolution at the present day."