ht on the very
order which nature has followed in the production of all the
existing animals; but it does not show why the structure of animals
in its increasing complexity from the more imperfect up to the most
perfect offers only an irregular gradation, whose extent presents a
number of anomalies or digressions which have no appearance of order
in their diversity.

"Now, in seeking for the reason of this singular irregularity in the
increasing complexity of organization of animals, if we should
consider the outcome of the influences that the infinitely
diversified circumstances in all parts of the globe exercise on the
general form, the parts, and the very organization of these animals,
everything will be clearly explained.

"It will, indeed, be evident that the condition in which we find all
animals is, on one side, the result of the increasing complexity of
the organization which tends to form a regular gradation, and, on
the other, that it is that of the influences of a multitude of very
different circumstances which continually tend to destroy the
regularity in the gradations of the increasing complexity of the
organization.

"Here it becomes necessary for me to explain the meaning I attach to
the expression _circumstances influencing the form and structure of
animals_--namely, that in becoming very different they change, with
time, both their form and organization by proportionate
modifications.

"Assuredly, if these expressions should be taken literally, I should
be accused of an error; for whatever may be the circumstances, they
do not directly cause any modification in the form and structure of
animals.

"But the great changes in the circumstances bring about in animals
great changes in their needs, and such changes in their needs
necessarily cause changes in their actions. Now, if the new needs
become constant or very permanent, the animals then assume new
_habits_, which are as durable as the needs which gave origin to
them. We see that this is easily demonstrated and even does not need
any explanation to make it clearer.

"It is then evident that a great change in circumstances having
become constant in a race of animals leads these animals into new
habits.

"Now, if new circumstances, having become permanent in a race of
animals, have given to these animals new _habits_--that is to say,
have led them to perform new