THE INNER TISSUES OF PLANTS. 291 



diately called forth by outer actions; but must have resulted 

 mediately through the effects of such outer actions on the 

 species. Let it be understood, therefore, that the differ- 

 entiations to which the foregoing interpretation applies, are 

 only those most conspicuous ones which are directly related 

 to the most conspicuous incident forces. They must be taken 

 as instances on the strength of wkich we may conclude that 

 other internal differentiations have had a natural genesis, 

 though in ways that we cannot trace. 



