by Richard Peachey
Both Lamarck and Darwin viewed evolution as slow and gradual. (Neither believed in "fixity of species.")
But their theories assigned different roles to the organism, and to the environment.
1. Jean-Baptiste de Lamarck (1744-1829)
Lamarck was a French scientist who, after the French Revolution, was appointed to be in charge of the invertebrate zoology section of the Paris Museum of Natural History. He wrote two major works, Philosophie zoologique (1809) and Histoire naturelle des animaux sans vertèbres (1815-1822).
Lamarck's theory of evolution has been summarized in two "laws":
(a) Law of Use and Disuse
The parts of an organism's body that are used become more developed; parts that are not used become smaller and may disappear.
(b) Law of Inheritance of Acquired Characteristics
Changes achieved over an organism's lifetime are passed on to its offspring. (Today this is considered to be the major flaw in Lamarck's reasoning, since we now know that only changes in an organism's sex cells will affect its offspring.)
Lamarck considered the environment as a stimulus causing organisms to strive to change, to make an effort to change — that is, organisms have an active role in their own evolution.
The classic example usually given in high school biology textbooks, though not a key example used by Lamarck, is of giraffes intentionally stretching to reach leaves at the tops of tall trees, then passing on any gains in neck length to their offspring. Over many generations, giraffes could achieve a much longer neck.
2. Charles Darwin (1809-1882)
Darwin's theory was developed in light of his five-year study of rocks, fossils, and living animals along the coast of South America and on islands in the Pacific Ocean. He wrote many books, of which the two most important are The Origin of Species (1859) and The Descent of Man (1871).
Darwin actually accepted Lamarck's two "laws" (see especially chapter 5 of The Origin of Species), but he generally laid greater stress on natural selection in his theory. The following are his main points:
Individuals within a population of organisms exhibit differences (they have traits that vary, such as colour, size, or running ability).
Due to limited resources (e.g., food, habitat, mates), there is usually competition among the individuals within a population of organisms (there is a struggle for existence). Populations cannot continue growing indefinitely (as discussed by Malthus).
(c) Elimination of the Less Fit
Within a population, individuals with less favourable traits will, on average, fail to survive and reproduce as often as those with more favourable traits. They will more often starve, or be killed by predators, or fail to attract mates (i.e., they will be "selected out").
(d) Survival of the Fittest
Individuals with more favourable traits will more often survive to reproductive age, will reproduce, and will transmit their favourable traits to their offspring. Over many generations, a higher percentage of the population will come to have the trait that is "selected for."
Darwin considered the environment as the selector in natural selection, removing the less fit and leaving the survivors to reproduce and predominate in the population. Organisms have only a passive role in their own evolution.
Using the giraffe example (though Darwin himself did not use it to argue against Lamarck), a Darwinist might say that longer and shorter necks existed in the ancestral population ("variation"), but only those with longer necks were able to reach food during times of scarcity ("competition"). The shorter-necked individuals were thus eliminated by the environment and did not survive to reproduce. Those with longer necks survived, reproduced, and passed on the long-neck trait to their offspring. Over many generations, the average neck length in the population therefore increased.
For further reading:
"Fixity of Species: A lesson in changing definitions" <http://www.answersingenesis.org/articles/2009/03/16/fixity-of-species>
"Darwin's Use of Lamarck's 'Laws'" <http://www.creationbc.org/index.php/darwins-use-of-lamarcks-laws/>
"The Giraffe: A Favourite Textbook Illustration of Evolutionary Theories" <http://www.creationbc.org/index.php/the-giraffe-a-favourite-textbook-illustration-of-evolutionary-theories/>
"Darwin's illegitimate brainchild" [on the historical fact that others, including creationists, recognized natural selection prior to Darwin] <http://creation.com/charles-darwins-illegitimate-brainchild>