e a multitude of examples of the effects of use
and disuse upon our own organs, effects which, if the use and disuse
were constant for many generations, would become much more marked.

"A great number of facts show, as will be more fully insisted on, that
when its will prompts an animal to this or that action, the organs which
are to execute it receive an excess of nervous fluid, and this is the
determinant cause of the movements necessary for the required action.
Modifications acquired in this way eventually become permanent in the
breed that has acquired them, and are transmitted to offspring, without
the offspring's having itself gone through the processes of acquisition
which were necessary in the case of the ancestor.[312] Frequent crosses,
however, with unmodified individuals, destroy the effect produced. It is
only owing to the isolation of the races of man through geographical and
other causes, that man himself presents so many varieties, each with a
distinctive character.

"A review of all existing classes, orders, genera, and species would
show that their structure, organs, and faculties, are in all cases
solely attributable to the surroundings to which each creature has been
subjected by nature, and to the habits which individuals have been
compelled to contract; and that they are not at all the result of a form
originally bestowed, which has imposed certain habits upon the
creature.[313]

"It is unnecessary to multiply instances; the fact is simply this, that
all animals have certain habits, and that their organization is always
in perfect harmony with these habits.[314] The conclusion hitherto
accepted is that the Author of Nature, when he created animals, foresaw
all the possible circumstances in which they would be placed, and gave
an unchanging organism to each creature, in accordance with its future
destiny. The conclusion, on the other hand, here maintained is that
nature has evolved all existing forms of life successively, beginning
with the simplest organisms and gradually proceeding to those which are
more complete. Forms of life have spread themselves throughout all the
habitable parts of the earth, and each species has received its habits
and corresponding modification of organs, from the influence of the
surroundings in which it found itself placed.[315]

"The first conclusion supposes an unvarying organism and unvarying
conditions. The second, which is my theory (_la mienne propre_),
suppose