ORGANIC REGULATION 97 
no mere “functional” change, but is also “structural,” 
as shown, for instance, by thickening of the epithelium. 
When structural elements are destroyed or actually 
removed, the process of reproduction is limited in the 
higher organisms. We then observe the phenomenon 
of other parts with similar function taking on the 
special functions of the lost part. Gradual recovery 
owing to other parts performing missing functions is a 
matter of everyday experience in Medicine and Sur- 
gery; and though the evidence is to a large extent still 
indirect, we cannot doubt that in all such cases struc- 
tural changes are associated with the functional adap- 
tation. The phenomena of vicarious function are also 
quite clearly adaptive changes, i.e., changes of such a 
nature that the life of the organism maintains as a 
whole its identity. 
When one regards the facts of memory from the 
purely physiological standpoint it is evident that 
memory is a phenomenon of the same nature as adap- 
tation. An experience or effort which has been gone 
through leaves its mark in the body as increased power 
of reaction to a similar experience or performance of a 
similar effort, just as an attack of an infectious disease, 
or vaccination, leaves its mark in a power of quickly 
repelling a similar infection. Were it not so memory 
would be a useless incumbrance. 
In this connection we may recall the facts relating 
to the effects of practice in the carrying out of any 
operation, such as writing, riding a bicycle, or playing 
a musical instrument. Here frequent repetition of 
what was at first a difficult and very imperfectly per- 
