In his _Philosophie Zoologique_, which appeared in 1809, Lamarck put forth his claim to regard all the phenomena of life, of living beings, and of man himself as pertaining to the domain of natural phenomena. According to him, all bodies which are met with in nature, organic and inorganic alike, are subject to the same laws. Life is a physical phenomenon, and all the processes of life are due to mechanical causes, either physical or chemical. He writes: "A leur source le physique et le moral ne sont sans doute qu'une seule et meme chose. Il faut rechercher dans la consideration de l'organisation les causes memes de la vie." In the intellectual evolution of the human mind perhaps no advance has been more important than that of Lamarck--the conquest of the domain of life by human intelligence. In conformity with the true scientific method, he founds his doctrine on the facts and phenomena of nature. "I confine myself," he says, "within the bounds of a simple contemplation {161} of nature." It was this observation of the gradual perfecting of living organisms from the simplest to the most complicated that inspired Lamarck with the idea of evolution and transformation. "How," he says, "can we help searching for the cause of such wonderful results? Are we not compelled to admit that nature has produced successively bodies endowed with life, proceeding from the simplest to the most complex?" The various products of nature have been divided into classes, genera, and species, simply to facilitate their study. Modern research tends to show that there is no definite line of demarcation even between the animal, vegetable, and mineral kingdoms. All our classification is artificial, and the passage from one division to another is gradual and insensible. Lamarck expresses this idea very clearly: "We must remember that classes, orders, and families, and all such nomenclature, are methods of our own invention. In nature there are no such things as classes or orders or families, but only individuals. As we become better acquainted with the productions of nature, and as the number of specimens in our collections increases, we see the intervals between the classes gradually fill up, and the lines of separation become effaced." Lamarck also raises his voice against the supposed immutability of species. "Species have only a relative constancy, depending on the circumstances of the individuals. The individuals of a given species perpetuate themselves without variation only so long as there is no variation in the circumstances which influence their existence. Numberless facts prove that when an individual of a given species changes its locality, it is subjected to a number of influences which little by little alter, not only the consistency and proportions of its parts, but also its form, its faculty, and even its organization; so that in time every part will participate in the mutations which it has undergone."