INFLUENCE OF ENVIRONMENT 123 



Those carnivores, for instance, which have become accustomed to 

 climbing, or to scratching the ground for digging holes, or to tearing 

 their prey, have been under the necessity of using the digits of their 

 feet : now this habit has promoted the separation of their digits, and 

 given rise to the formation of the claws with which they are armed. 



But some of the carnivores are obliged to have recourse to pursuit 

 in order to catch their prey : now some of these animals were compelled 

 by their needs to contract the habit of tearing with their claws, which 

 they are constantly burying deep in the body of another animal in 

 order to lay hold of it, and then make efforts to tear out the part seized. 

 These repeated efforts must have resulted in its claws reaching a size 

 and curvature which would have greatly impeded them in walking or 

 running on stony ground : in such cases the animal has been compelled 

 to make further efforts to draw back its claws, which are so pro- 

 jecting and hooked as to get in its way. From this there has gradually 

 resulted the formation of those pecuhar sheaths, into which cats, 

 tigers, lions, etc. withdraw their claws when they are not using them. 



Hence we see that efforts in a given direction, when they are long 

 sustained or habitually made by certain parts of a living body, for 

 the satisfaction of needs established by nature or environment, cause 

 an enlargement of these parts and the acquisition of a size and shape 

 that they would never have obtained, if these efforts had not become 

 the normal activities of the animals exerting them. Instances are 

 everywhere furnished by observations on all known animals. 



Can there be any more striking instance than that which we find 

 in the kangaroo ? This animal, which carries its young in a pouch 

 under the abdomen, has acquired the habit of standing upright, so 

 as to rest only on its hind legs and tail ; and of moving only by means 

 of a succession of leaps, during which it maintains its erect attitude 

 in order not to disturb its young. And the following is the result : 



1. Its fore legs, which it uses very little and on which it only supports 

 itself for a moment on abandoning its erect attitude, have never 

 acquired a development proportional to that of the other parts, and 

 have remained meagre, very short and with very little strength. 



2. The hind legs, on the contrary, which are almost continually 

 in action either for supporting the whole body or for making leaps, 

 have acquired a great development and become very large and strong. 



3. Lastly, the tail, which is in this case much used for supporting 

 the animal and carrying out its chief movements, has acquired an 

 extremely remarkable thickness and strength at its base. 



These well-known facts are surely quite sufficient to establish the 

 results of habitual use on an organ or any other part of animals. If 

 on observing in an animal any organ particularly well-developed. 



