Lamarck's theory of evolution is worked out in great detail in his _Philosophie zoologique_, but the exposition is diffuse and disconnected; it is better in giving an account of it to follow the more concise, mature and general exposition which he gives in the Introduction to his _Histoire naturelle des Animaux sans Vertebres_.[341] Near the beginning of the Introduction Lamarck gives us in a few short "Fundamental Principles" the main lines of his general philosophy. He is a confirmed materialist. Every fact and phenomenon is essentially physical and owes its existence or production entirely to material bodies or to relations between them. All change and all movement is in the last resort due to mechanical causes. Every fact or phenomenon observed in a living body is at once a physical fact or phenomenon and a product of organisation (p. 19). Life, thought and sensation are not properties of matter, but result from particular material combinations. His thorough-going materialism is most clearly shown in its relation to living things in the first three of the "Zoological Principles and Axioms," which are developed further on in the book. These are as follows:--"1. No kind or particle of matter can have in itself the power of moving, living, feeling, thinking, nor of having ideas; and if, outside of man, we observe bodies endowed with all or one of these faculties, we ought to consider these faculties as physical phenomena which Nature has been able to produce, not by employing some particular kind of matter which itself possesses one or other of these faculties, but by the order and state of things which she has constituted in each organisation and in each particular system of organs. "2. Every animal faculty, of whatever nature it may be, is an organic phenomenon, and results from a system of organs or an organ-apparatus which gives rise to it and upon which it is necessarily dependent. "3. The more highly a faculty is developed the more complex is the system of organs which produces it, and the higher the general organisation; the more difficult also does it become to grasp its mechanism. But the faculty is none the less a phenomenon of organisation, and for that reason purely physical" (p. 104). According to these "axioms" function is a direct and mechanical effect of structure. The curious thing is that in spite of his avowed materialism, Lamarck's conception of life and evolution is profoundly psychological, and from the conflict of his materialism and his vitalism (of which he was himself hardly conscious), arise most of the obscurities and the irreductible self-contradiction of his theory.