100 THE BEGINNINGS OF LIFE. 



and structures which are definitely related to the 

 functions and structures of other parts. The whole is 

 composed of parts working in accord with one another, 

 and in such a manner as to establish a harmony 

 between the actions going on within and those without 

 the organism. The result of this interaction during 

 past time has been the gradual elaboration of an 

 organism of a certain structure ; so that this structure 

 and form are only to be regarded as the physical 

 expression of an approximate equilibrium between 

 numerous related factors between the inherent ten- 

 dencies of c physiological units ' under the influence of 

 all past c conditions,' and the present operation of 

 external forces upon the now-acquired structure. When, 

 therefore, unaccustomed conditions act upon such or- 

 ganisms, they are unable easily or within short periods 

 to produce direct modifications of the organs principally 

 affected, because a change in one important organ would 

 necessitate other changes throughout the whole organism, 

 in order to establish a new balance of functions. This, 

 in fact, is the only conclusion which seems consistent 

 with doctrines of Evolution. Mr. Spencer says: c lf 

 we assume, as we must according to this hypothesis, 

 that the structure of any organism is the product of the 

 almost infinite series of actions and reactions to which 

 all ancestral organisms have been exposed, we shall 

 see that any unusual actions and reactions brought 

 to bear on an individual can have but an infinitesimal 

 effect in permanently changing the structure of the 



