Though he was building on the work of his mentor,
Count George-Louis Leclerc
de Buffon, Jean-Baptiste Lamarck
(1744-1829) is
often credited with making the first large advance toward modern evolutionary
theory because he was the first to propose a mechanism by which the gradual
change of species might take place. Also, he extended the definition of the
change over time, saying that life started out simple and became more complex.
In 1809 he published
Philosophie Zoologique, in which he described a two
part mechanism by which change was gradually introduced into the species and
passed down through generations. His theory is alternatively referred to as the
theory of transformation or simply
Lamarckism. Though today Lamarck's work
is considered a major step forward, in his lifetime he did not receive much
recognition.
Use and Disuse
Figure2.1: Use and disuse in the evolution of the neck of the giraffe
The classic example used to explain the concept of
use and disuse is the
elongated neck of the giraffe. According to Lamarck's theory, a given giraffe
could, over a lifetime of straining to reach high branches, develop an elongated
neck. A major downfall of his theory was that he could not explain how this
might happen, though he discussed a "natural tendency toward perfection."
Another example Lamarck used was the toes of water birds. He proposed that from
years of straining their toes to swim through water, these birds gained
elongated, webbed toes to better their swimming.
These two examples demonstrate how use could change a trait. By the same token,
Lamarck believed that disuse would cause a trait to become reduced. The wings of
penguins, for example, would be smaller than those of other birds because
penguins do not use them to fly.
Lamarckian Inheritance
The second part of Lamarck's mechanism for evolution involved the inheritance
of acquired traits. He believed that traits changed or acquired over an
individual's lifetime could be passed down to its offspring. Giraffes that had
acquired long necks would have offspring with long necks rather than the short
necks their parents were born with. This type of inheritance, sometimes called
Lamarckian inheritance, has since been disproved by the discovery
of hereditary genetics.
An extension of Lamarck's ideas of inheritance that has stood the test of time,
however, is the idea that evolutionary change takes place gradually and
constantly. He studied ancient seashells and noticed that the older they were,
the simpler they appeared. From this, he concluded that species started out
simple and consistently moved toward complexity, or, as he termed it, closer to
perfection.
