970 Influence of circumstances on the Actions of Animals. 
every day, to change all that which she continues to produce, they 
are in many ways inexhaustible to her. The principal proceed 
from the influence of climates, from that of diverse temperatures 
of atmosphere, and of all the surrounding media; that of the 
diversity of places, and of their situation; that of habits, of the 
. most ordinary movements, of the most frequent actions; finally, of 
that of the means of self-preservation, of manner of life, of defence, 
of multiplication, ete. Now, by continuance of these diverse influ- 
ences the faculties become expanded and strengthened by use, 
become diversified by new habits long maintained, and insensibly 
the conformation, consistence, in a word, the nature and condition 
of parts, also that of organs, participate in all these influences, 
maintaining and propagating themselves by generation. 
These truths, which are only the result of the two natural laws 
expounded above, are in all-cases strictly confirmed by facts ; they 
indicate clearly the progress of nature in the diversity of her pro- 
ductions. But in place of contenting ourselves with generalities 
which may be considered as hypothetical, let us examine strictly 
the facts, and consider, in animals, the result of the use or disuse 
of their organs upon these organs themselves. According to the 
habits which each race has been forced to contract, I will prove 
that the constant lack of exercise, as regards an organ, diminishes 
at first its functions, gradually impoverishes it, and in the end 
makes it disappear, or annihilates it; so this disuse perpetuates 
itself for a long time thereafter in successive generations of animals 
of the same race. Afterwards I will show that, on the contrary, 
the habitual exercise of an organ in any animal which has not 
experienced a diminution of its functions, not only perfects and 
increases its functions, but otherwise makes it acquire the develop- 
ment and dimensions which insensibly change it, so that after a 
time it renders it different from the corresponding organ of another 
animal which exercises it much less. : 
The disuse of an organ having resulted in the habits which it 
has assumed, gradually impoverishes the organ, and finally makes 
it disappear, or obliterates it. As such a proposition could 
be admitted only on proof, and not on simple declaration, we will 
attempt to make it evident by the citation of known facts which 
constitutes its basis. 
