ploys every day to bring about variations in all that she
continues to produce, we can say that they are in her in some degree
inexhaustible.

"The principal ones arise from the influence of climate, from that
of different temperatures, of the atmosphere, and from all
environing surroundings (_milieux_); from that of the diversity of
places and their situations; from that of the most ordinary habitual
movements, of actions the most frequent; finally from that of the
means of preservation, of the mode of life, of defence, of
reproduction, etc.

"Moreover, as the result of these different influences the faculties
increase and strengthen themselves by use, diversify themselves by
the new habits preserved through long periods, and insensibly the
conformation, the consistence--in a word, the nature and state of
the parts and also of the organs--consequently participate in all
these influences, are preserved and propagate themselves by
generation" (_Systeme des Animaux sans Vertebres_, p. 12).

* * * * *

"It is easy for any one to see that the habit of exercising an organ
in every living being which has not reached the term of diminution
of its faculties not only makes this organ more perfect, but even
makes it acquire developments and dimensions which insensibly change
it, with the result that with time it renders it very different from
the same organ considered in another organism which has not, or has
but slightly, exercised it. It is also very easy to prove that the
constant lack of exercise of an organ gradually reduces it and ends
by atrophying it."

Then follow the facts regarding the mole, spalax, ant-eater, and the
lack of teeth in birds, the origin of shore birds, swimming birds and
perching birds, which are stated farther on.

"Thus the efforts in any direction, maintained for a long time or
made habitually by certain parts of a living body, to satisfy the
needs called out (_exiges_) by nature or by circumstances, develop
these parts and cause them to acquire dimensions and a form which
they never would have obtained if these efforts had not become an
habitual action of the animals which have exercised them.
Observations made on all the animals known would furnish examples of
this.

"When the will determines an animal to any kind of action, the
organs whose function it is to execute this action are then