ii6 THEORETICAL BIOLOGY 



different functions, such as walking, running and leaping, 

 and so are not concentrated on one function alone, as 

 the leaping foot of the kangaroo is concentrated, down to 

 the last detail. Considered superficially, they perhaps seem 

 to be imperfect attempts to approximate to the function 

 exercised at the moment ; in reality, considering the changes 

 that the subsidiary functions demand of the organism, per- 

 fection is always achieved. 



Accordingly, we are not justified in comparing the various 

 forms assumed by an organ with the gradual perfecting of our 

 own implements, which are always merely approximately 

 adequate to the counter-action. 



But, by renouncing this comparison, we lose a very import- 

 ant piece of evidence, which has been adduced to prove the 

 gradual evolution of organs, evidence that is especially con- 

 vincing because the analogy seems so obvious. 



The organs of animals are always the perfect expression 

 of one function or of several, and consequently changes that 

 take place in them point to a change of function. The func- 

 tions themselves, however, are always unities, and not subject 

 to change. One function, it is true, may more or less force 

 another into the background or even cause it to disappear 

 altogether ; but functions themselves do not change. 



For instance, I can take up a half-sitting, half-lying 

 position, but it is as impossible for me to lie more or lie less, 

 as to sit more or sit less. 



SCHEMA AND FUNCTION 



Again and again we can demonstrate that, when our eyes 

 are shut and the order is given that the left hand is to write 

 a certain letter quickly, it is very apt to lapse into mirror- 

 writing ; and this does not happen if we guide the pen slowly. 

 By closing the eyes and writing quickly, the control is removed 



