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. 
