THE INNER TISSUES OF ANIMALS. 361 



contractile tissue and other tissues, have been caused by ana- 

 logous unlikenesses. 



304. These interpretations, which have already occupied 

 too large a space, must here be closed. Of course out of 

 phenomena so multitudinous and varied, it has been imprac- 

 ticable to deal with any but the most important; and it haa 

 been practicable to deal with these only in a general way. 

 Much, however, as remains to be explained, I think the possi- 

 bility of tracing, in so many cases, the actions to which these 

 internal differentiations may rationally be ascribed, makes it 

 likely that the remaining internal differentiations are due to 

 kindred actions. We find evidence that in more cases 

 than seemed probable, these actions produce their effects 

 directly on the individual ; and that the unlikenesses are 

 produced by accumulation of such effects from generation to 

 generation. While for the remaining unlikenesses, we have, 

 as an adequate cause, the indirect effects wrought by the sur- 

 vival, generation after generation, of the individuals in which 

 favourable variations have occurred variations such as those 

 of which human anatomy furnishes endless instances. Thus 

 accounting for so much, we may not unreasonably presume 

 that these co-operative processes of direct and indirect equili- 

 bration will account for what remains. 



Though not strictly included under the title of the chap- 

 ter, there is a subject on which a few words may here be 

 added, because of the elucidations yielded to it by some 

 parts of the chapter. I refer to the repair and growth of the 

 differentiated tissues. When treating inductively of that resto- 

 ration which takes place in worn organs, it was admitted that 

 little in the way of deductive interpretation is apparent 

 nothing beyond the harmony between the facts and the 

 general principle of segregation ( 64). And it was further 

 admitted that it is not obvious why, within certain limits, an 

 organ grows in proportion as it is exercised. Certain of the 

 foregoing considerations, however, help us towards a partial 



