968 Influence of circumstances on the Actions of Animals. 
“interior sentiment.” This is the question, as I will presently 
prove by known facts. To arrive at a knowledge of the true causes 
of so many diverse forms and so many different habits, of which 
known animals offer us examples, it is necessary to consider that 
the infinitely diversified circumstances, but slowly changing, which 
the animals of each race are continually encountering, produce for 
each of them new wants and necessarily changes in their habits, 
Now this incontestable truth once acknowledged, it will be 
easy to perceive how these new wants could be satisfied, and these 
new habits assumed, if we give some attention to the two following 
laws of nature, which observation has always proved to be 
constant :— 
First Law.—In every animal which has not passed the time of 
its development the frequent and sustained employment of an organ — 
gradually strengthens it, developes and enlarges it, and gives it 
power proportional to the duration of its use; whilst the constant 
disuse of a like organ weakens it, insensibly deteriorates it, pro- 
gressively reduces its functions, and finally causes it to disappear. 
Second Law.—All that nature acquires or loses in individuals by 
the influence of circumstances to which the race has been exposed 
for a long time, and in consequence by the influence of the predom- 
inate employment of such organ, or by the influence of disuse of 
such part, she preserves by generation, among new individuals which 
spring from it, providing the acquired changes be common to both 
sexes, or to those which have produced new individuals. 
These are, then, two constant truths which cannot be miscon- 
strued, except by those who have never observed or followed nature 
in her operations, or by those who entertain an error which I will 
combat. Naturalists having remarked that the forms of the parts 
of animals are always perfectly in harmony with the use of those 
parts, have thought that the forms and the conditions of the parts 
had caused their employment. Now this is an error, for it is easy tO 
demonstrate by observation that on the contrary it is the wants and 
uses of the parts which have developed these same parts, that they 
are made to exist where they did not, and that consequently they 
have given place to the condition in which we observe them in 
every animal. 
For, had this not been so, it would be necessary that nature 
