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.' 



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 diflFerent 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 1. 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 organization attains its maximum. Com- 

 plexity of structure necessarily carries with it complexity 

 of function, and in proportion as distinct functions 



* 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.) 



