_species_ among organized beings?

"When we consider the series of animals, beginning at the end
comprising the most perfect and complicated, and passing down
through all the degrees of this series to the other end, we see a
very evident modification in structure and faculties. On the
contrary, if we begin with the end which comprises animals the most
simple in organization, the poorest in faculties and in organs--in a
word, the most imperfect in all respects--we necessarily remark, as
we gradually ascend in the series, a truly progressive complication
in the organization of these different animals, and we see the
organs and faculties of these beings successively multiplying and
diversifying in a most remarkable manner.

"These facts once known present truths which are, to some extent,
eternal; for nothing here is the product of our imagination or of
our arbitrary principles; that which I have just explained rests
neither on systems nor on any hypothesis: it is only the very simple
result of the observation of nature; hence I do not fear to advance
the view that all that one can imagine, from any motives whatever,
to contradict these great verities will always be destroyed by the
evidence of the facts with which it deals.

"To these facts it is necessary to add these very important
considerations, which observation has led me to perceive, and the
basis of which will always be recognized by those who pay attention
to them; they are as follows:

"Firstly, the exercise of life, and consequently of organic
movement, constitutes its activity, tends, without ceasing, not only
to develop and to extend the organization, but it tends besides to
multiply the organs and to isolate them in special centres
(_foyers_). To make sure whether the exercise of life tends to
extend and develop the organization, it suffices to consider the
state of the organs of any animal which has just been born, and to
compare them in this condition with what they are when the animal
has attained the period when its organs cease to receive any new
development. Then we will see on what this organic law is based,
which I have published in my _Recherches sur les Corps vivans_
(p. 8), _i.e._, that--

"'The special property of movement of fluids in the supple parts of
the living body which contain them is to open (_frayer_) there
routes, places of deposit and tissues; to create