RECENT CRITICISMS AND HYPOTHESES. 571 



ones are fertile), it is reasonable to infer that minute and 

 obscure alterations of this kind may make slightly-different 

 varieties unable to inter-breed. 



Granting that there goes on this " physiological selection," 

 we must recognize it as one among the causes by which isola- 

 tion is produced, and the differentiating influence of natural 

 selection in the same locality made possible. 



§ 174^. The foregoing criticisms and hypotheses do not, 

 however, affect in any essential way the pre-existing concep- 

 tions. If, as in the foregoing chapters, we interpret the facts 

 in terms of that redistribution of matter and motion consti- 

 tuting Evolution at large, we shall see that the general theory, 

 as previously held, remains outstanding. 



It is indisputable that to maintain its life an organism 

 must maintain the moving equilibrium of its functions in 

 presence of environing actions. This is a truism : overthrow 

 of the equilibrium is death. It is a corollary that when the 

 environment is changed, the equilibrium of functions is dis- 

 turbed, and there must follow one of two results — either the 

 equilibrium is overthrown or it is re-adjusted: there is a 

 re-equilibration. Only two possible ways of effecting the 

 re-adjustment exist — the direct and the indirect. In the one 

 case the changed outer action so alters the moving equili- 

 brium as to call forth an equivalent reaction which balances 

 it. If re-equilibration is not thus effected in the individual 

 it is effected in the succession of individuals. Either the 

 species altogether disappears, or else there disappear, genera- 

 tion after generation, those members of it the equilibria of 

 whose functions are least congruous with the changed actions 

 in the environment ; and this is the survival of the fittest or 

 natural selection. 



If now we persist in thus contemplating the problem as a 

 statico-dynamical one, we shall see that much of the discus- 

 sion commonly carried on is beside the question. The centre 

 around which the collision of arguments has taken place, is 



