INTERACTION OF HEREDITY AND ADAPTATION. 219 



in the two sets of phenomena being so completely inter- 

 crossed and interwoven. We are but seldom able to say 

 with certainty — of the variations of form which occur before 

 our eyes — how much is owing to Inheritance, and how much 

 to Adaptation. All characters of form, by which organisms 

 are distinguished, are caused either by Inheritance or by 

 Adaptation ; but as both functions are continually inter- 

 acting with each other, it is extremely difficult for the 

 systematic inquirer to recognize the share belonging to each 

 of the two functions in the special structure of individual 

 forms. This is, at present, all the more difficult, because we 

 are as yet scarcely aware of the immense importance of this 

 fact, and because most naturalists have neglected the theory 

 of Adaptation, as well as that of Inheritance. The laws of 

 Inheritance, which we have just discussed, as well as the 

 laws of Adaptation, which we shall consider directly, in 

 reality form only a small portion of the phenomena existing 

 in this domain, but which have not as yet been investi- 

 gated ; and since every one of these laws can interact with 

 every other, it is clear that there is an infinite complication 

 of physiological actions, which are at work in the con- 

 struction of organisms. 



But now, as to the phenomenon of variation or adaptation 

 in general, we must, as in the case of inheritance, view it as 

 a quite universal, physiological fundamental quality of all 

 organisms, without exception — as a manifestation of life 

 which cannot be separated from the idea of organism. 

 Strictly speaking, we must here also, as in the case of in- 

 heritance, distinguish between Adaptation itself and Adapta- 

 bility. By Adaptation (Adaptio), or Variation ( Variatio), we 

 understand the fact that the organism, in consequence of 



