ADAPTATION 173 



the changes of the mechanical conditions according to the 

 medium also seem to have some sort of structural effect. 

 If plants stand very deeply in water, the conditions of 

 illumination, so important for assimilation in plants, may 

 have been altered, and therefore much of the structural 

 change can be attributed also to them. It is unimportant 

 in our general question what is due to one of these factors 

 and what to the other. That there is a real sort of 

 adaptation cannot be doubtful ; and the same is true, as 

 experimental observations of the last few years have shown, 

 with regard to the structural differences between so-called 

 sun-leaves and shade-leaves of plants grown in the air : 

 it has been actually shown here that the functional life 

 of the former goes on better in the sun, of the latter better 

 in the shade. 



It is very important to emphasise this point, as the 

 adaptive character of all sorts of structural differences in 

 plants dependent on light and on moisture has lately been 

 denied, on the supposition that there is only a stopping of 

 organogenesis in the case of the more simple, a continuance 

 in the case of the more complicated modification, but 

 nothing else. Indeed, all morphological adaptation has 

 been conceived as only consisting in differences dependent 

 upon the absence or the presence of necessary means or 

 causes of development, and as offering no problem of its 

 own. We have gained the right position from which to 

 oppose this argument, it seems to me, in our formula that 

 all adaptations do relate not directly to the agents of the 

 medium, but to changes of functional states induced by 

 those agents ; that adaptations only are " adaptations ' by 

 being correctives to the functional state. 



