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Beginning in 1801, Lamarck began to publish details of his evolutionary theories. Where men like Buffon had hinted at the possibility of evolutionary change, Lamarck declared it forthrightly. In 1801 he wrote:
. . . time and favorable conditions are the two principal means which nature has employed in giving existence to all her productions. We know that for her time has no limit, and that consequently she always has it at her disposal.
What was the mechanism for evolution? "Lamarckism" or "Lamarckianism" is now often used in a rather derogatory sense to refer to the theory that acquired traits can be inherited. What Lamarck actually believed was more complex: organisms are not passively altered by their environment, as his colleague Geoffroy Saint-Hilaire thought. Instead, a change in the environment causes changes in the needs of organisms living in that environment, which in turn causes changes in their behavior. Altered behavior leads to greater or lesser use of a given structure or organ; use would cause the structure to increase in size over several generations, whereas disuse would cause it to shrink or even disappear. This rule -- that use or disuse causes structures to enlarge or shrink -- Lamarck called the "First Law" in his book Philosophie zoologique. Lamarck's "Second Law" stated that all such changes were heritable. The result of these laws was the continuous, gradual change of all organisms, as they became adapted to their environments; the physiological needs of organisms, created by their interactions with the environment, drive Lamarckian evolution. Lamarck intended, is in conflict with the findings of genetics and has now been largely abandoned -- but until the rediscovery of Mendel's laws at the beginning of the twentieth century, no one understood the mechanisms of heredity, and Lamarckian inheritance was a perfectly reasonable hypothesis. Several other scientists of the day, including Erasmus Darwin, subscribed to the theory of use and disuse -- in fact, Erasmus Darwin's evolutionary theory is so close to Lamarck's in many respects that it is surprising that, as far as is known now, the two men were unaware of each other's work.
In several other respects, the theory of Lamarck differs from modern evolutionary theory. Lamarck viewed evolution as a process of increasing complexity and "perfection," not driven by chance; as he wrote in Philosophie zoologique, "Nature, in producing in succession every species of animal, and beginning with the least perfect or simplest to end her work with the most perfect, has gradually complicated their structure." Lamarck did not believe in extinction: for him, species that disappeared did so because they evolved into different species. If this goes on for too long, it would mean the disappearance of less "perfect" organisms; Lamarck had to postulate that simple organisms, such as protists, were constantly being spontaneously generated. Yet despite these differences, Lamarck made a major contribution to evolutionarythought, developing a theory that paralleled Darwin's in many respects. Rediscovered in the middle part of the 19th century, his theories finally gained the attention they merited.