318 LAMARCK, HIS LIFE AND WORK 
had not become the habitual action of the animals 
which have exercised them. The observations made 
on all the animals known will everywhere furnish 
examples. 
“Can any of them be more striking than that which 
the kangaroo offers us? This animal, which carries its 
young in its abdominal pouch, has adopted the habit 
of hol lding itself erect, standing only on its hind feet 
and tail, and only changing its position by a series of 
leaps, in which it preserves its erect attitude so as not 
to injure its young. 
Bet wsrsee tine cecilia: 
“1, Its fore legs, of which it makes little use, and 
on which it rests only during the instant when it 
leaves its erect attitude, have never reached a de- 
velopment proportionate to that of the other parts, 
and have remained thin, very small, and weak; 
“2. The hind legs, almost continually in action, 
both for supporting the body and for leaping, have, 
on the contrary, obtained a considerable develop- 
ment, and have become very large and strong; 
“3, Finally, the tail, which we see is of much use 
in Seine the animal and in the performance of 
its principal movements, has acquired at its base a 
thickness and a strength extremely remarkable. 
“These well-known facts are assuredly well calcu- 
lated to prove what results from the habitual use in 
the animals of any organ or part; and if, when there 
is observed in an animal an organ especially well de- 
veloped, strong, and powerful, it is supposed that its 
habitual use has not produced it, that its continual 
disuse will make it lose nothing, and, finally, that this 
organ has always been such since the creation of the 
species to which this animal belongs, I will ask why 
our domestic ducks cannot fly like wild ducks—in a 
word, I might cite a multitude of examples which 
prove the differences in us resulting from the exercise 
or lack of use of such of our organs, although these 
