THE BEGINNINGS OF LIFE. 95 



trariwise, the aggregate is made by incident actions to 

 take a new form, its forces must tend to remould the 

 units into harmony with this new form. And to say 

 that the physiological units are in any degree so re- 

 moulded as to bring their polar forces towards equi- 

 librium with the forces of the modified aggregate, is 

 to say that when separated in the shape of reproduc- 

 tive centres, the units will tend to build themselves 

 up into an aggregate modified in the same direction. 3 



Amongst simple organisms almost any part of the sub- 

 stance which separates, or is separated, from one of them 

 is capable of developing into a similar simple organism. 

 But as organisms grow more and more complex in their 

 structure, so we find that a difference arises in the re- 

 productive powers of different tissues till at last the 

 capacity to reproduce the entire organism (either with- 

 out fertilization or only after this has occurred) becomes 

 restricted to the morphological units which are produced 

 in special organs \ How much this restriction of the 

 reproductive function is due to a general specialization 

 is obvious from the fact that it is most marked where 

 complexity of organisation attains its maximum. Com- 

 plexity of structure necessarily carries with it complexity 

 of function, and in proportion as distinct functions 



1 The necessity for the fertilization of some of these reproductive ele- 

 ments, and the evolution of sexual differences amongst the animals and 

 plants amongst which this necessity obtains, is merely a superadded 

 complexity a difference of degree and not of kind. The fundamental 

 phenomena of reproduction are essentially similar in sexual and sexless 

 organisms. (See Spencer's ' Principles of Biology,' vol. i. pp. 218-223.) 



