Lamarck and Species Evolution

Jean Baptiste Lamarck (1744-1829) was first trained as a military officer. However, after a fall on his head, he quit that career to take up his real passion: botany. His first work, on the flora of France, appeared in 1778, and he was invited to Paris by the leading natrualist, Georges Buffon, to work at the Royal Garden of Plants. Following the French revolution, the Royal Garden was reorganized as the Museum of Natural History, and professorships opened in a wide variety of fields. Lamarck applied for the prestigious professorship in vertebrate zoology, but was not successful; instead, he was offered the less prestigious professorship of the invertebrates -- containing the catch all category of "worms" that Linnaeus had included in his earlier classification.

Lamarck eventually separated out no less than 10 sub-divisions within the invertebrates, ranging from the Infusorians (with the simplest nervous system and internal organs) through insects to the molluscs (the most complex of the invertebrates -- e.g. shrimps, crabs, and lobsters). Within the vertebrates, he distinguished four classes - the fish, reptiles, birds, and mammals (as in Linnaeus); to which he added, between the reptiles and birds, the class of "monotremes" to include the newly discovered "duck-billed platypus" from Australia (today included as a mammal) -- which combined the bird-like habit of laying eggs, in an animal which provided milk to its young. This system was set out in the multi-volumed History of Animals without Backbones published from 1815-1822.

Although most respected among his colleagues for that work, our interest will focus on an earlier, more speculative work: the Philosophical Zoology of 1809. Here, for the first time in biology (the term "biology" was coined by Lamarck to include zoology and botany) a theory of species change was adopted as a central concept. What Lamarck did was to take the classes he had defined in his systematics and reconceptualize them as a sequence of transformations of some into others over time. The following diagram from his Zoological Philosophy illustrates this concept:

For our purposes the main thing to note is the trail of downward lines, indicating descent from earlier, simpler classes to later, more complex ones. The details of the various classes are secondary, as are the details of how this affects each species within a class. Note at the bottom of the descending lines the following mammalian classes:

Lamarck developed a theory which identified three factors to account for this evolutionary process:

  1. Life itself originated through "spontaneous generation" of the simplest organisms (worms) from non-living or dead material. This process was a continuous, on-going one: at any time, more elementary forms of life were being generated. The doctrine of spontaneous generation was widely held in the first half of the 19th century, and not refuted until Pasteur's experiment ofthe second half of the century, where he showed that maggots and the like arose in decaying meat because of invisible eggs deposited on the carrion by flies, not some spontaneous generation.

  2. Once generated, species evolved through a "perfecting principle" innate to life itself. Lamarck believed that there was an inner nature or essence of life which drove it to become more complex, as measured by the organization of the nervous system and the internal organs: later, more complex organisms had more centralized nervous systems and more differentiated internal organs. This perfecting principle did not rest on any physical mechanism, a point which would lead many (including Darwin) to reject this theory of evolution as "metaphysical", not scientific.

  3. Each species had to deal with its immediate environment. If that environment were to substantially change, the individuals of that species would have to change as well in order to remain viable. They would struggle to adapt themselves to the modified environment, either consciously (in animals with brains), or unconsciously (the rest). The modifications made to their bodies to develop faculties needed for survival were inherited by their progeny, permanently modifying the species. Lamarck called this process "use-inheritance" or "inheritance of acquired characteristics". Modern biologists have found no evidence for such a biological process.

Taken together, Lamarck's three factors constitute his theory of evolution. You can read his own description of the theory here, beginning with a discussion of the evolution of the body of the seal. But taken individually, each of his factors is false. Lamarck was a creative thinker who deserves a major place in the history of biology. He challenged the theory of species fixity; but he could not complete the scientific revolution begun by Vesalius, Harvey, and Linnaeus. This was due to the excessively speculative nature of his theory, especially the "perfecting factor", which does not provide an explanation meeting the canons of scientific discourse: he had no physical mechanism, only a metaphysical claim. Darwin will attempt to change that with his theory of evolution chiefly, though not solely, via "natural selection".