INFLUENCE OF ENVIRONMENT 126 



and species of existing animals, I should be able to show that the 

 conformation and structure of individuals, their organs, faculties, 

 etc., etc., are everywhere a pure result of the environment to which 

 each species is exposed by its nature, and by the habits that the 

 individuals composing it have been compelled to acquire ; I should 

 be able to show that they are not the result of a shape which existed 

 from the beginning, and has driven animals into the habits they are 

 known to possess. 



It is known that the animal called the at or sloth (Bradypustridactylus) 

 is permanently in a state of such extreme weakness that it only executes 

 very slow and limited movements, and walks on the ground with 

 difficulty. So slow are its movements that it is alleged that it can only 

 take fifty steps in a day. It is known, moreover, that the organisation 

 of this animal is entirely in harmony with its state of feebleness and 

 incapacity for walking ; and that if it wished to make other movements 

 than those which it actually does make it could not do so. 



Hence on the supposition that this animal had received its organisa- 

 tion from nature, it has been asserted that this organisation forced 

 it into the habits and miserable state in which it exists. 



This is very far from being my opinion ; for I am convinced that 

 the habits which the ai was originally forced to contract must 

 necessarily have brought its organisation to its present condition. 



If continual dangers in former times have led the individuals of 

 this species to take refuge in trees, to live there habitually and feed 

 on their leaves, it is clear that they must have given up a great number 

 of movements which animals living on the ground are in a position 

 to perform. All the needs of the ai will then be reduced to clinging 

 to branches and crawling and dragging themselves among them, 

 in order to reach the leaves, and then to remaining on the tree in a 

 state of inactivity in order to avoid falling off. This kind of inactivity, 

 moreover, must have been continually induced by the heat of the 

 climate ; for among warm-blooded animals, heat is more conducive 

 to rest than to movement. 



Now the individuals of the race of the ai have long maintained this 

 habit of remaining in the trees, and of performing only those slow 

 and Uttle varied movements which suffice for their needs. Hence 

 their organisation will gradually have come into accordance with 

 their new habits ; and from this it must follow : 



1. That the arms of these animals, which are making continual 

 efforts to clasp the branches of trees, will be lengthened ; 



2. That the claws of their digits will have acquired a great length 

 and a hooked shape, through the continued efforts of the animal to 

 hold on ; 



