1062 Influence of circumstances on the Actions of Animals. 
Behold, then, what is the result? First.—Its front legs, of which it 
makes very little use, and upon which it supports itself only an 
instant when it leaves its upright attitude, have never acquired a 
development proportional to those of other parts, and have remained 
slender, very small, and almost without strength. Second.—The hind 
limbs, almost continually in action to sustain all the body, when leap- 
ing, have, on the contrary, attained a considerable development, 
and have become large and strong. Third.—Finally, the tail, which 
we'see greatly employed in sustaining the animal, and in executing 
its principal movements, has acquired at its base a breadth and a 
force extremely remarkable. These well-known facts are assuredly 
well calculated to prove that which results from the habitual use by 
animals of an organ or of some part. If, when we observe in an 
animal an organ particularly developed, and strong and powerful, — 
it is claimed that its habitual exercise has done nothing to produce 
that result ; that its continued.disuse makes it lose nothing, and that, 
finally, this organ has always been such as we find it since the 
creation of the species to which this animal belongs, I demand why 
our domestic ducks are not able to fly as the wild ducks; in a word, 
I will bring a multitude of examples to our notice, which will attest 
the differences resulting to us from the exercise or the lack of exer- 
cise of some of our organs, although these differences be not maim- 
tained in successive generations. In that case their results might 
be still more considerable. I observe, in the second place, that 
when the will determines an animal to some action, the organs 
which should execute this action are immediately excited by the 
influence of subtle fiuids (of the nervous fluid), which becomes the 
determining cause of the movements which cause the action m 
question. A multitude of observations prove this fact. It 
results that the multiplied repetitions of these acts of organiza- 
tion strengthen, expand, develop, and also create the organs which 
are necessary. It is necessary only to observe attentively that which 
happens everywhere in this respect, to be convinced of the basis of 
_ this cause of the development of organic changes. A 
Now, all changes acquired in an organ in consequence of a habit 
employed sufficiently to have an effect, is preserved afterward by 
~ generation, if it is common to the individuals who in fecundation 
_ unite for the reproduction of their species. Finally this change 15 
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