certain explanations which would prevent any one from
misunderstanding them." It may be inferred from this that he had for
some time previous meditated on this theme. It will now be interesting
to see what factors of evolution Lamarck employed in this first sketch
of his theory.

After stating the distinctions existing between the vertebrate and
invertebrate animals, and referring to the great diversity of animal
forms, he goes on to say that Nature began with the most simply
organized, and having formed them, "then with the aid of much time and
of favorable circumstances she formed all the others."

"It appears, as I have already said, that _time_ and _favorable
conditions_ are the two principal means which nature has employed in
giving existence to all her productions. We know that for her time
has no limit, and that consequently she has it always at her
disposal.

"As to the circumstances of which she has had need and of which she
makes use every day in order to cause her productions to vary, we
can say that they are in a manner inexhaustible.

"The essential ones arise from the influence and from all the
environing media (_milieux_), from the diversity of local causes
(_diversite des lieux_), of habits, of movements, of action, finally
of means of living, of preserving their lives, of defending
themselves, of multiplying themselves, etc. Moreover, as the result
of these different influences the faculties, developed and
strengthened by use (_usage_), became diversified by the new habits
maintained for long ages, and by slow degrees the structure, the
consistence, in a word the nature, the condition of the parts and of
the organs consequently participating in all these influences,
became preserved and were propagated by generation.[163]

"The bird which necessity (_besoin_) drives to the water to find
there the prey needed for its subsistence separates the toes of its
feet when it wishes to strike the water[164] and move on its
surface. The skin, which unites these toes at their base, contracts
in this way the habit of extending itself. Thus in time the broad
membranes which connect the toes of ducks, geese, etc., are formed
in the way indicated.

"But one accustomed to live perched on trees has necessarily the end
of the toes lengthened and shaped in another way. Its claws are
elongated, sharpened, and are curved and bent so as to seize the
branches