INFLUENCE OF ENVIRONMENT 115 



imperceptibly affected by these influences and are preserved and 

 propagated by reproduction. 



These truths, which are merely effects of the two natural laws 

 stated above, receive in every instance striking confirmation from 

 facts ; for the facts afford a clear indication of nature's procedure in 

 the diversity of her productions. 



But instead of being contented with generalities which might be 

 considered hypothetical, let us investigate the facts directly, and 

 consider the effects in animals of the use or disuse of their organs 

 on these same organs, in accordance with the habits that each race 

 has been forced to contract. 



Now I am going to prove that the permanent disuse of any organ 

 first decreases its functional capacity, and then gradually reduces 

 the organ and causes it to disappear or even become extinct, if this 

 disuse lasts for a very long period throughout successive generations 

 of animals of the same race. 



I shall then show that the habit of using any organ, on the con- 

 trary, in any animal which has not reached the limit of the decHne 

 of its functions, not only perfects and increases the functions of that 

 organ, but causes it in addition to take on a size and development 

 which imperceptibly alter it ; so that in course of time it becomes 

 very different from the same organ in some other animal which uses 

 it far less. 



The 'permanent disuse of an organ, arising from a change of habits, 

 causes a gradual shrinkage and ultimately the disappearance and even 

 extinction of that organ. 



Since such a proposition could only be accepted on proof, and not 

 on mere authority, let us endeavour to make it clear by citing the chief 

 known facts which substantiate it. 



The vertebrates, whose plan of organisation is almost the same 

 throughout, though with much variety in their parts, have their jaws 

 armed with teeth ; some of them, however, whose environment has 

 induced the habit of swallowing the objects they feed on without any 

 preliminary mastication, are so affected that their teeth do not develop. 

 The teeth then remain hidden in the bony framework of the jaws, 

 without being able to appear outside ; or indeed they actually become 

 extinct down to their last rudiments. 



In the right-whale, which was supposed to be completely destitute 

 of teeth, M. Geoffroy has nevertheless discovered teeth concealed 

 in the jaws of the foetus of this animal. The professor has moreover 

 discovered in birds the groove in which the teeth should be placed, 

 though they are no longer to be found there. 



