Lamarck was the first to formulate as a scientific theory the natural
origin of living things, including man, and to push the theory to its
extreme conclusions--the rise of the earliest organisms by spontaneous
generation (or abiogenesis) and the descent of man from the nearest
related mammal, the ape. He sought to explain this last point, which
is of especial interest to us here, by the same agencies which he
found at work in the natural origin of the plant and animal species.
He considered use and habit (adaptation) on the one hand, and heredity
on the other, to be the chief of these agencies. The most important
modifications of the organs of plants and animals are due, in his
opinion, to the function of these very organs, or to the use or disuse
of them. To give a few examples, the woodpecker and the humming-bird
have got their peculiarly long tongues from the habit of extracting
their food with their tongues from deep and narrow folds or canals;
the frog has developed the web between his toes by his own swimming;
the giraffe has lengthened his neck by stretching up to the higher
branches of trees, and so on. It is quite certain that this use or
disuse of organs is a most important factor in organic development,
but it is not sufficient to explain the origin of species.

To adaptation we must add heredity as the second and not less
important agency, as Lamarck perfectly recognised. He said that the
modification of the organs in any one individual by use or disuse was
slight, but that it was increased by accumulation in passing by
heredity from generation to generation. But he missed altogether the
principle which Darwin afterwards found to be the chief factor in the
theory of transformation--namely, the principle of natural selection
in the struggle for existence. It was partly owing to his failure to
detect this supremely important element, and partly to the poor
condition of all biological science at the time, that Lamarck did not