great problem is, How is it that existing animals, and, as the evidence of
the remains of extinct animals shows, these that existed at former periods
of time also, are divided into the groups or types we call species,
naturally classified into larger groups which are subdivisions of others
still larger, and so on, in what we call the natural system of
classification? The two problems which naturalists have to solve, and
which for many recent generations they have been trying to solve, are the
Origin of Species and the Origin of Adaptations.

Former generations of zoologists have assumed that these problems were the
same. Lamarck maintained that the peculiarities of different animals were
due to the fact that they had become adapted to modes of life different to
those of their ancestors, and to those in which allied forms lived, the
change of structure being due to the effect of the conditions of life and
of the actions of the organs. He did not specially consider the
differences of closely allied species, but the peculiarities of marked
types such as the long neck of the giraffe, the antlers of stags, the
trunk of the elephant, and so on; but he considered that the action of
external conditions was the true cause of evolution, and assumed that in
course of time the effects became hereditary.

Lamarck's views are expounded chiefly in his _Philosophie Zoologique_,
first published in 1809, and an excellent edition of this work with
biographical and critical introduction was published by Charles Martins in
1873. Although his conception of the mode in which structural changes were
produced is of little importance to those now engaged in the investigation
of the process of evolution, since it was naturally based on the
physiological ideas of his time, many of which are now obsolete, for the
sake of accuracy it is worth while to cite his principal propositions in
his own words:--