78 Darwin, and after Darwin. 



the animal, whereby the whole body is enabled con- 

 tinually to readjust its balance on a book (or any 

 other plane surface), as this is slowly rotated on 

 a horizontal axis. So long as the book is lying flat, 

 the frog remains motionless ; but as soon as the book 

 is tilted a little, so that the frog is in danger of 

 slipping off, all the four feet begin to crawl up the 

 hill ; and the steeper the hill becomes, the faster 

 they crawl. When the book is vertical, the frog 

 has reached the now horizontal back, and so on. 

 Such being the facts, the question is How can the 

 complicated piece of machinery thus implied have 

 been developed by natural selection? Obviously it 

 cannot have been so by any of the parts concerned 

 having been originally distributed among different 

 individuals, and afterwards united in single individuals 

 by survival (i.e. free intercrossing) of the fittest. 

 In other words, the case is obviously one of co-adap- 

 tation, and not one of the blending of adaptations. 

 Again, and no less obviously, it is impossible that 

 the co-adaptation can have been gradually developed 

 by natural selection, because, in order to have been 

 so, it must by hypothesis have been of some degree 

 of use in every one of its stages ; yet it plainly 

 cannot have been until it had been fully perfected 

 in all its astonishing complexity 1 . 



1 Of course it will be observed that the question is not with regard 

 to the development of all the nerves and muscles concerned in this 

 particular process. It is as to the development of the co-ordinating 

 centres, which thus so delicately respond to the special stimuli furnished 

 by variations of angle to the horizon. And it is as inconceivable in this 

 cnse of reflex action, as it is in almost every other case of reflex action, 

 that the highly specialized machinery required for performing the adaptive 

 function can ever have had its origin in the performance of any other 



