78 CAUSES OF VARIATION. [introd. 



be compounded together in the middle line forming a single, 

 symmetrical organ. 



It would probably help the science of Biology if the word 

 'Reversion' and the ideas which it denotes, were wholly dropped, 

 at all events until Variation has been studied much more fully 

 than it has yet been. 



In the light of what we now know of the process of repro- 

 duction the phrase is almost meaningless. We suppose that a 

 certain stock gives off a number of individuals which vary about 

 a normal ; and that after having given them off, it begins to 

 give off individuals varying about another normal. We want 

 to say that among these it now and then gives off one which 

 approaches the first normal, that shooting at the new mark it 

 now and then hits the old one. But all that we know is that 

 now and then it shoots wide and hits another mark, and we 

 assume from this that it would not have hit it if it had not 

 aimed at it in a bygone age. To apply this to any other matter 

 would be absurd. We might as well say that a bubble would 

 not be round if the air in it had not learned the trick of round- 

 ness by having been in a bubble before : that if in a bag after 

 pulling out a lot of white balls I find a totally red one, this 

 proves that the bag must have once been full of red balls, or that 

 the white ones must all have been red in the past. 



Besides the logical absurdity on which this use of the theory 

 of Reversion rests, the application of it to the facts of Variation 

 breaks down again and again. I have already mentioned some 

 cases of this, but there are many others of a different class. For 

 instance, it will be shewn that the percentage of extra molars 

 in the Anthropoid Apes is almost the highest reached among 

 mammals. On the usual interpretation, such teeth are due to 

 Reversion to an ancestral condition with 4 molars, and on less 

 evidence it has been argued that a form frequently shewing such 

 " Reversion " is older than those which do not. From this reason- 

 ing it should follow that the Anthropoids are the most primitive 

 form, at least of monkeys. It is surely time that these brilliant 

 and facile deductions were no more made in the name of science. 



3. Causes of Variation. 



Inquiry into the causes of Variation is as yet, in my judgment, 

 premature. 



4. The Variability of " useless " Structures. 



The often-repeated statement that " useless " parts are 

 especially variable, finds little support in the facts of Variation, 

 except in as far as it is a misrepresentation of another principle. 

 The examples taken to support this statement are commonly 

 organs standing at the end of a Meristic Series of parts, in which 



