Organic Evolution. 183 



that we may say that the modification of life is in the 

 direction of more or of less, complete adaptation to simpler 

 or to more complex conditions. Where the adaptation 

 advances to more complex conditions, we speak of elabora- 

 tion ; where it retrogrades to less complex conditions, we 

 speak of degeneration; but both fall under the head of 

 evolution in its more general sense. Viewed as a whole, 

 there can be little doubt that the general tendency of 

 evolution is towards more complete adaptation to more 

 diverse and complex environment. And this tendency is 

 accompanied by a general increase of differentiation and 

 of integration ; of differentiation whereby the constituent 

 elements of life, whether cells, tissues, organs, organisms, 

 or groups of organisms, become progressively more 

 specialized and more different from one another; of 

 integration whereby these elements become progressively 

 more interdependent one on the other. We may con- 

 veniently sum up the tendency towards more perfect 

 adaptation to more complex circumstances in the word 

 progress; the tendency to differentiation in the word 

 individuality; and the tendency to integration in the 

 word association. 



Nobody now doubts the propositions thus briefly sum- 

 marized, and it is therefore unnecessary to bring forward 

 evidence in their favour. 



We may pass, then, to the question How ? Evolution 

 being continuity, associated with change, tending in certain 

 directions, and accompanied by certain processes, how has 

 it been effected ? What are its methods ? 



Natural Selection. 



Natural selection claims a foremost place. We have 

 already devoted a chapter to its consideration. Animals 

 vary ; more are born than can survive to procreate their 

 kind ; hence a struggle for existence, in which the weaker 

 and less adapted are eliminated, the stronger and better 

 adapted surviving to continue the race. 



