Organic Evolution. 2 1 1 



say — I do not believe that use and disuse have had anything 

 whatever to do with the matter. Fortuitous variations in 

 these digits have taken place. The conditions have deter- 

 mined which variations should be preserved. In the horse, 

 variations in the direction of increase of functional value 

 of the mid digit, and variations in the simultaneous 

 decrease of the functional value of the lateral digits, have 

 been of advantage, and have therefore survived the elimi- 

 nating process of natural selection. 



Now, since it is quite clear, in this and numberless 

 similar cases, that we can explain the facts either way, it 

 is obviously not worth while to spend much time or 

 ingenuity in devising such explanations. They are not 

 likely to convince any one worth convincing. What we 

 need is (1) crucial cases which can only be explained one 

 way or the other ; or (2) direct observation or experiment 

 leading to the establishment of one hypothesis or the other 

 (or both). 



1. Crucial cases are very difficult to find. We cannot 

 exclude the element of use or disuse, for on both hypotheses 

 it is essential. The difference is that one school says the 

 organ is developed in the species by use ; the other school 

 says it is developed for use. What we must seek is, there- 

 fore, the necessary exclusion of natural selection ; and that 

 is not easy to prove, in any case, to a Darwinian. If it can 

 be shown that there exist structures which are of use, but 

 not of vital importance (that is to say, which have not 

 what I called above the available advantage necessary to 

 determine the question of elimination or not-elimination), 

 then we are perhaps able to exclude the influence of natural 

 selection. I think, if anywhere, such cases are to be found 

 in faculties and instincts ; * and as such they must be 

 considered in a later chapter. I will, however, here cite 

 one case in illustration of my meaning. 



* I find, on rereading one of his articles, that I have here unwittingly- 

 adopted one of Mr. Romanes's arguments (see Nature, vol. xxxvi. p. 406). 

 The instance Mr. Romanes cites is the curious habit of dogs turning round 

 before they lie down. 



