328 THEORETICAL BIOLOGY 



he took hold of the stick at various places, until he managed 

 to jump through with it when he held it by the crook, for 

 now the stick no longer presented an obstacle. 



Undoubtedly it is of very great help in biology to seek out 

 typical examples, because only through such can we get 

 visual representation. But we must carefully analyse the 

 typical case down to its ultimate elements, if it is to serve 

 as an explanation of other cases. 



In Jennings' example, it is obvious that the dog behaves 

 differently from a machine. A driverless motor-car, for 

 instance, would have smashed the stick, or would have come 

 to a standstill in front of the gap in the fence. So in a certain 

 sense we may say that here we have a typical example of a 

 " living " action, as opposed to a mechanical. 



We might just as well take the case of a fly, which comes 

 to a window-pane, hits it with its head several times, and 

 then no longer treats it as though it were air, but walks about 

 on it as if on the ground. 



In both instances, through the coming in of an indication, 

 a rearrangement of the action is undertaken. 



But Jennings' experiment tells us nothing further. Rightly 

 understood, it suggests that all the function-actions of animals 

 are determined by indications, while those of machines are 

 not. In so far, it is typical. But there is not the slightest 

 ground for trying to make it hold good in the case of develop- 

 mental actions. And just as little will it serve as evidence of 

 adaptation : for both the old and the new indications were 

 foreseen in the organisation of the dog and of the fly, and 

 the new actions also lay within the normal one conditioned 

 thereby. Moreover, the nervous steering-gear that undertakes 

 the change was already present in both cases. 



Meanwhile Jennings himself became convinced that the 

 comparison of the genesis of the organism with the behaviour 

 of the dog is open to objection. For through experiments 



