DIVISION OF LABOUR. 247 
the organs themselves, and thus we find, for example, many 
crabs, or crustacea, which in their youth possess a tolerably 
high degree of organization, viz. legs, antennze, and eyes, in 
old age completely degenerate, living as parasites, with- 
out eyes, without apparatus of motion, and without antenne. 
The lively, active form of youth, has become a shapeless, 
motionless lump. Only the most necessary organs of nutri- 
tion and propagation retain their activity; all the rest of 
the body has degenerated. Evidently these complete trans- 
formations are, to a large extent, the direct consequences of 
cumulative adaption, of the non-use and defective exercise 
of the organs, but a great portion of them must certainly 
be attributed also to correlative adaptation. (Compare Plate 
X. and XI). 
A seventh law of adaptation, the fourth in the group of 
direct adaptation, is the law of divergent adaptation. By 
this law we indicate the fact that parts originally formed 
alike have developed in different ways under the influence 
of external conditions. This law of adaptation is extremely 
important for the explanation of the phenomenon of 
division of labour, or polymorphism. We can see this 
very easily in our own selves; for instance, in the activity 
of our two hands. We usually accustom our right hand 
to quite different work from that which we give our left, 
and in consequence of the different occupation there arises 
a different formation of the two hands. The right hand, 
which we use much more than the left, shows a stronger 
development of the nerves, muscles, and bones. The same 
applies to the whole arm. In most human beings the 
bones and flesh of the right arm are, in consequence 
of their being more employed, stronger and heavier than 
