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, antennse, and eyes, in 

 old age completely degenerate, living as parasites, with- 

 out eyes, without apparatus of motion, and without antennae. 

 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; aU 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 diiSerent 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 



