LAMARCK’S THEORY OF DESCENT 321 
vinced that the habits which the individuals of the 
race of the az were originally compelled to contract 
have necessarily brought their organization into its 
actual state. 
‘Since continual exposure to dangers has at some 
time compelled the individuals of this species to take 
refuge in trees and to live in them permanently, and 
then feed on their leaves, it is evident that then they 
would give up making a multitude of movements 
that animals which live on the ground perform. 
“ All the needs of the az would then be reduced to 
seizing hold of the branches, to creeping along them or 
to drawing them in so as to reach the leaves, and then 
to remain on the tree in a kind of inaction, so as to 
prevent falling. Besides, this kind of sluggishness 
would be steadily provoked by the heat of the 
climate; for in warm-blooded animals the heat urges 
them rather to repose than to activity. 
“ Moreover, during a long period of time the indi- 
viduals of the race of the az having preserved the 
habit of clinging to trees and of making only slow 
and slightly varied movements, just sufficient for their 
needs, their organization has gradually become adapted 
to their new habits, and from this it will result: 
“1, That the arms of these animals making con- 
tinual efforts readily to embrace the branches of trees, 
would become elongated; 
“2, That the nails of their digits would acquire 
much length and a hooked shape, by the continued 
efforts of the animal to retain its hold; 
“3, That their digits never having been trained to 
make special movements, would lose all mobility 
among themselves, would become united, and would 
only preserve the power of bending or of straighten- 
ing out all together; 
“4. That their thighs, continually embracing both 
the trunks and the larger branches of trees, would 
contract a condition of habitual separation which 
