82 Darwin, and after Darwin. 



of all the other general principles, which have just 

 been applied in the two preceding cases. But it is 

 only one of these other general principles which 

 I desire in the present case specially to consider, 

 for the purpose of considering more closely than 

 hitherto the difficulty which this principle presents 

 to ultra-Darwinian theory. 



The difficulty to which I allude is that of under- 

 standing how all the stages in the development of 

 a reflex action can have been due to natural selection, 

 seeing that, before the reflex mechanism has been 

 sufficiently elaborated to perform its function, it can- 

 not have presented any degree of utility. Now the 

 particular force of the present example, the action 

 of scratching as also of the one to follow consists 

 in the fact that it is a case where a reflex action is 

 not yet completely organized. It appears to be only 

 in course of construction, so that it is neither in- 

 variably present, nor, when it is present, is it ever 

 fully adapted to the performance of its function. 



That it is not invariably present (when the brain 

 is so) may be proved by trying the simple experi- 

 ment on a number of puppies and also of full- 

 grown dogs. Again, that even when it is present 

 it is far from being fully adapted to the perform- 

 ance of its function, may be proved by observing 

 that only in rare instances does the scratching 

 leg succeed in scratching the place which is being 

 irritated. The movements are made more or less at 

 random, and as often as not the foot fails to touch 

 the body at any place at all. Hence, although we 

 have a " prophecy " of a reflex action well designed 

 for the discharge of a particular function, at present 



