PROGRESS, BIOLOGICAL AND OTHER 29 



metazoan individuals, communities), secondly by 

 their aggregation; and this has been accompanied by 

 a (very roughly) parallel increase in the duration of 

 life. 



Next, there has been an increase in their complex- 

 ity; and this in its turn depends upon the fact that 

 a division of labour has been brought about between 

 the parts of organisms, each part becoming special- 

 ized for greater efficiency in the performance of some 

 particular function. In the fewest words, the sepa- 

 rate bits of machinery of which organisms are com- 

 posed have become more efficient. 



In the third place, there has been an increase in the 

 harmony of these parts, and consequently in the 

 unity of the whole. Delicate mechanisms for co- 

 ordination have been developed, and arrangements 

 whereby one portion becomes dominant over the rest, 

 and so a material basis for unification is given. 



In the fourth place, there has been an increase of 

 self-regulation. The outer environment changes 

 from month to month, from hour to hour. The more 

 complex products of evolution are in high degree 

 exempt from the consequences of these changes, 

 through being the possessors of a constant internal 

 environment which, beyond the narrowest limits, it 

 is most difficult to alter. 



Fifthly, there has been an increase in the possi- 

 bility of bringing past experience to bear on present 

 problems. At the base is the power of modifying 

 normal reactions with repetition; then come some 



