THE ORIGIN OF ORGANS 109 



aquatic, the limb that was used for terrestrial progression 

 is modified back into a functional fin ; and again, when 

 flight becomes necessary, the same organ is used for the 

 new function. So that whatever the changes in the mode 

 of progression, we need no new organ at all ; for the old 

 organ is used for the new purpose. It is very much 

 easier to understand how a useful level can be attained 

 in that way than by organs starting ab initio. But of 

 course we must come down to a true beginning if we 

 push our inquiries far enough. In attempting this, we 

 are carried to those remote times in which the ancestors 

 of vertebrates arose. Upon these forms we can do no 

 more than speculate, but it is at any rate impossible to 

 prove that bud-like projections from the body, probably 

 later reduced to four, may not have been useful, from 

 their very beginning, to a slender worm-like animal for 

 pushing its way through mud or thick weeds, or for the 

 purpose of respiration. Professor E. B. Tylor has told 

 me that he believes that the same thing holds with regard 

 to human weapons. He said that, in examining ancient 

 weapons, he was often struck with the fact that a weapon 

 or implement had ultimately turned out to be so very 

 much more useful for a new purpose rather than that for 

 which it was originally formed. Here, then, one origin 

 apparently accounts for several forms of implement. 



Another objection raised against Natural Selection is 

 that a selective cause is never a true cause. Professor 

 Cope means to imply that when he speaks of the ' Origin 

 of the fittest '. But Darwin's argument on this point is 

 perfectly sufficient. He says that when a man drops 

 iron into sulphuric acid, he does not originate the chemical 

 force that operates, but he may be fairly said to make 

 sulphate of iron. So Natural Selection does not itself 

 originate the factors upon which it depends, but it is so 

 essential to the result that it may be fairly looked upon 

 as the true cause (at that level of causation). In Galton's 

 work we have a most complete inquiry into human 

 variation and its inheritance, and he shows us that such 

 variation by itself, unguided by selection, can never 

 advance to anything. Even if you start with ancestors 



