262 NATURAL HISTORY. 



will increase both in size and weight, and will at the 

 same time become more fully competent to discharge 

 its particular work. On the other hand, disuse of a 

 muscle leads, in the first place, to a decreased ability 

 on the part of the organ to perform its proper function. 

 Thus, the muscles of the left hand cannot in ordinary 

 people be used as efficiently as those of the right 

 hand, because they are less often used. If the disuse 

 be prolonged and habitual, the organ will become 

 diminished in size, and the ability to discharge its 

 function may be wholly lost. Thus, man possesses the 

 three muscles which are attached to the external ear, 

 and which enable many of the quadrupeds to move 

 their ears freely ; but in him they are extremely reduced 

 in size, and the power of employing them has, from 

 disuse, become almost, or entirely, lost. Finally, if 

 the disuse of an organ be complete, as when external 

 conditions no longer demand its employment, it may 

 become a mere rudiment, absolutely destitute of function. 

 This is the case, for example, with the eyes of certain 

 animals which spend their existence living underground 

 and in total darkness. Lamarck fully recognised the 

 important results which flow from the use or disuse 

 of organs ; and it was, therefore, to the effects of habit 

 that he chiefly ascribed the progressive modifications 

 which he believed to have affected the structure of all 

 living beings. He believed that a change in the sur- 

 rounding conditions would necessarily compel an animal 

 to modify its former habits of life; and that a change 

 in its habits would necessarily entail an increased use 

 of certain organs and a decreased use of others. The 



