While he was working on
Hydrogéologie (1802), Lamarck had the idea to apply the principle of
erosion to biology. This led him to the basic principle of evolution, which saw the fluids in organs inheriting more complex forms and functions, thus passing on these traits to the organism's descendants. This was a reversal from Lamarck's previous view, published in his
Memoirs of Physics and Natural History (1797), in which he briefly refers to the immutability of species. Lamarck stressed two main themes in his biological work (neither of them to do with soft inheritance). The first was that the environment gives rise to changes in animals. He cited examples of blindness in moles, the presence of teeth in mammals and the absence of teeth in birds as evidence of this principle. The second principle was that life was structured in an orderly manner and that many different parts of all bodies make possible the organic movements of animals. Although he was not the first thinker to advocate organic evolution, he was the first to develop a truly coherent evolutionary theory. He outlined his theories regarding evolution first in his
Floreal lecture of 1800, and then in three later published works: • ''Recherches sur l'organisation des corps vivants'', 1802. •
Philosophie zoologique, 1809. •
Histoire naturelle des animaux sans vertèbres, (in seven volumes, 1815–22). Lamarck employed several mechanisms as drivers of evolution, drawn from the common knowledge of his day and from his own belief in the chemistry before
Lavoisier. He used these mechanisms to explain the two forces he saw as constituting evolution: force driving animals from simple to complex forms and a force adapting animals to their local environments and differentiating them from each other. He believed that these forces must be explained as a necessary consequence of basic physical principles, favoring a materialistic attitude toward biology.
Le pouvoir de la vie: The complexifying force s towards higher levels (
orthogenesis) creating a ladder of
phyla, and 2) an adaptive force that causes animals with a given body plan to adapt to circumstances (use and disuse,
inheritance of acquired characteristics), creating a diversity of
species and
genera. Popular views of Lamarckism consider only an aspect of the adaptive force. Lamarck referred to a tendency for organisms to become more complex, moving "up" a
ladder of progress. He referred to this phenomenon as
Le pouvoir de la vie or ''la force qui tend sans cesse à composer l'organisation'' (The force that perpetually tends to make order). Lamarck believed in the ongoing
spontaneous generation of simple living organisms through action on physical matter by a material life force. Lamarck ran against the modern chemistry promoted by Lavoisier (whose ideas he regarded with disdain), preferring to embrace a more traditional alchemical view of the elements as influenced primarily by earth, air, fire, and water. He asserted that once living organisms form, the movements of fluids in living organisms naturally drove them to evolve toward ever greater levels of complexity: {{blockquote|The rapid motion of fluids will etch canals between delicate tissues. Soon their flow will begin to vary, leading to the emergence of distinct organs. The fluids themselves, now more elaborate, will become more complex, engendering a greater variety of secretions and substances composing the organs. He argued that organisms thus moved from simple to complex in a steady, predictable way based on the fundamental physical principles of alchemy. In this view, simple organisms never disappeared because they were constantly being created by spontaneous generation in what has been described as a "steady-state biology". Lamarck saw spontaneous generation as being ongoing, with the simple organisms thus created being transmuted over time becoming more complex. He is sometimes regarded as believing in a
teleological (goal-oriented) process where organisms became more perfect as they evolved, though as a materialist, he emphasized that these forces must originate necessarily from underlying physical principles. According to the paleontologist
Henry Fairfield Osborn, "Lamarck denied, absolutely, the existence of any 'perfecting tendency' in nature, and regarded evolution as the final necessary effect of surrounding conditions on life."
Charles Coulston Gillispie, a historian of science, has written "life is a purely physical phenomenon in Lamarck", and argued that Lamarck's views should not be confused with the
vitalist school of thought.
''L'influence des circonstances'': The Adaptive Force The second component of Lamarck's theory of evolution was the
adaptation of organisms to their environment. This could move organisms upward from the ladder of progress into new and distinct forms with local adaptations. It could also drive organisms into evolutionary blind alleys, where the organism became so finely adapted that no further change could occur. Lamarck argued that this adaptive force was powered by the interaction of organisms with their environment, by the use and disuse of certain characteristics.
First law: use and disuse :First Law: In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly becomes weak and deteriorates it, and progressively diminishes its functional capacity or ability to function as expected, until it finally disappears.
Second law: inheritance of acquired characteristics :Second Law: All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young. The last clause of this law introduces what is now called
soft inheritance, the inheritance of acquired characteristics, or simply "Lamarckism", though it forms only a part of Lamarck's thinking. However, in the field of
epigenetics, evidence is growing that soft inheritance plays a part in the changing of some organisms' phenotypes; it leaves the genetic material (
DNA) unaltered (thus not violating the
central dogma of biology) but prevents the expression of
genes, such as by
methylation to modify
DNA transcription; this can be produced by changes in behaviour and environment, though there is no known example in which this is related to the use or disuse of an organ or a function. Many epigenetic changes are heritable to a degree, though often only for few generations. Thus, while DNA itself is not directly altered by the environment and behavior except through selection, the relationship of the genotype to the phenotype can be altered, even across some generations, by the surrounding within the lifetime of an individual. This has led to calls for biologists to reconsider the possibility of Lamarckian-like processes in evolution in light of modern advances in molecular biology. ==Religious views==