Lamarck's Scientific Thought

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. While the mechanism of Lamarckian evolution is quite different from that proposed by Darwin, the predicted result is the same: adaptive change in lineages, ultimately driven by environmental change, over long periods of time. It is interesting to note that Lamarck cited in support of his theory of evolution many of the same lines of evidence that Darwin was to use in the Origin of Species. Lamarck's Philosophie zoologique mentions the great variety of animal and plant forms produced under human cultivation (Lamarck even anticipated Darwin in mentioning fantail pigeons!); the presence of vestigial, non-functional structures in many animals; and the presence of embryonic structures that have no counterpart in the adult. Like Darwin and later evolutionary biologists, Lamarck argued that the Earth was immensely old. Lamarck even mentions the possibility of natural selection in his writings, although he never seems to have attached much importance to this idea. It is even more interesting to note that, although Darwin tried to refute the Lamarckian mechanism of inheritance, he later admitted that the heritable effects of use and disuse might be important in evolution. In the Origin of Species he wrote that the vestigial eyes of moles and of cave-dwelling animals are "probably due to gradual reduction from disuse, but aided perhaps by natural selection." Lamarckian inheritance, at least in the sense 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.



Bibliography:http://www.ucmp.berkeley.edu/history/lamarck.html