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 
