12 FUTILITY OF THIS METHOD. [introd. 



The real objection however to the employment of the Study of 

 Adaptation as a means of discovering the processes of Evolution is 

 not that its results are meagre and its conclusions unsound. Apart 

 from the doubtful character of these inferences, there is a difficulty 

 of logic which in this method is inherent and insuperable. This 

 difficulty lies in the fact that while it is generally possible to 

 suggest some way by which in circumstances, known or hypothe- 

 tical, any given structure may be of use to any animal, it cannot on 

 the other hand ever be possible to prove that such structures are 

 not on the whole harmful either in a way indicated or otherwise. 

 There is a special reason why the impossibility of proving the 

 negative applies with peculiar force to the mode of reasoning we 

 are now considering. This is due to the fact that whereas the 

 only possible test of the utility of a structure must be a quan- 

 titative one, such a quantitative method of assessment is entirely 

 beyond our powers and is likely to remain so indefinitely. The 

 students of Adaptation forget that even on the strictest applica- 

 tion of the theory of Selection it is unnecessary to suppose that 

 every part an animal has, and every thing which it does, is useful 

 and for its good. We, animals, live not only by virtue of, but also 

 in spite of what we are. It is obvious from inspection that any 

 instinct or any organ may be of use : the real question we have to 

 consider is of how much use it is. To know that the presence of a 

 certain organ may lead to the preservation of a race is useless if we 

 cannot tell how much preservation it can effect, how many indi- 

 viduals it can save that would otherwise be lost; unless we kn<»w 

 also the degree to which its presence is harmful ; unless, in fact, we 

 know how its presence affects the profit and loss account of the 

 organism. We have no right to consider the utility of a structure 

 demonstrated, in the sense that we may use this demonstration as 

 evidence of the causes which have led to the existence of the struc- 

 ture, until we have this quantitative knowledge of its utility and are 

 able to set off against it the cost of the production of the structure 

 and all the difficulties which its presence entails on the organism. 

 X<> one who has ever tried to realize the complexity of the relations 

 between an organism and its surroundings, the infinite variety of 

 the consequences which every detail of structure and every shade of 

 instinct may entail upon the organism, the precision of the correla- 

 tion between function and the need for it, and above all the mar- 

 vellous accuracy with which the- presence or absence of a power or 

 a structure is often compensated among living beings — no one can 

 reflect upon these things and be hopeful that our quantitative 

 estimates of utility are likely to be correct. But in the absence of 

 such correct and final estimates of utility, we must never use the 

 utility of a structure as a point of departure in considering the 

 manner of its origin ; for though we can see that it is, or may be, 

 useful, yet a little reflexion will shew that it is, or may be, harmful, 

 but whether on the whole it is useful or on the whole harmful, 



