I 



ADAPTATION IS UNLIMITED. 249 



acquire a completely different form and extent, a completely 

 different degree of curvature and diameter of spiral winding, 

 according as they twine themselves round a thinner or a 

 thicker bar. The divergent change of form of parts origin- 

 ally identical in form, which tending in different directions 

 develop themselves under different external conditions, can 

 be distinctly demonstrated in many other examples. As 

 this divergent adaptation interacts with progressive inherit- 

 ance, it becomes the cause of a division of labour among the 

 different organs. 



An eighth and last law of adaptation we may call the 

 law of unlimited or infinite adaptation. By it we simply 

 mean to express that we know of no limit to the variation 

 of organic forms occasioned by the external conditions of 

 existence. We can assert of no single part of an organism, 

 that it is no longer variable, or that if it were subjected to 

 new external conditions it would not be changed by them. 

 It has never yet been proved by experience that there is a 

 limit to variation. If, for example, an organ degenerates 

 from non-use, this degeneration ends finally in a complete 

 disappearance of the organ, as is the case with the eyes of 

 many animals. On the other hand, we are able, by continual 

 practice, habit, and the ever-increasing use of an organ, to 

 bring it to a degree of perfection which we should at 

 the beginning have considered to be impossible. If we com- 

 pare the uncivilized savages with civilized nations, we find 

 among the former a development of the organs of sense — 

 sight, smell, and hearing — such as civilized nations can 

 hardly conceive of On the other hand, the brain, that is 

 mental activity, among more civilized nations is developed 

 to a degree of which the wild savages have no idea. 



