THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 49 
more and more pronounced. Among the more complex 
polyps and jelly-fishes the contractile cells form distinct 
muscle fibers and muscles; the sensitive cells form dis- 
tinct nerve cells and nerve fibers which are arranged in a 
primitive nervous system; the digestive cavity becomes 
complex and composed of different portions ; the reproduc- 
tive cells are formed by special organs, and the distinction 
between the egg cells and the sperm cells—that is, be- 
tween the female reproductive elements and the male 
reproductive elements—becomes more pronounced. 
We have followed this increase or development of struc- 
tural and physiological complexity from simplest animals 
to fairly complex ones. The principle of this development 
of complexity is evident. It will not be profitable to at- 
tempt to follow in detail this development among the 
higher animals. The complex animals are complex be- 
cause their life processes are performed by special parts of 
their body, which parts are specially modified so as to perform 
these processes well. The animals which are more complex 
than those we have studied differ from these simply in the 
degree of complexity attained. In order to understand 
this better we shall not further consider special groups of 
animals, but special processes or functions, and attempt to 
see how the modification and increase in complexity of 
structure goes hand in hand with the increase of elaborate- 
ness or complexity in the performance of function. 
