230 THE VARIATION OF ANIMALS IN NATURE 



populations. This appears to hold whether they define the 

 conditions governing the spread of new mutations or of new 

 combinations. The theory of the evolution of dominance 

 has also been considered. It seems at present to lack 

 sufficient direct verification, while some of the indirect evidence 

 is of doubtful value. 



? IV. Indirect Evidence for and against the Natural 



Selection Theory. — We have seen that the direct evidence 

 for a selective process is inadequate both in quality and 

 quantity. This inadequacy is largely due to the difficulties 

 involved in the necessary investigations. Recent work on 

 insect parasites and some of the fishery investigations suggest 

 that the direct method of attack is not so hopeless as has been 

 thought. Under the stimulus of economic gain — e.g. in the 

 Cornborer investigations — it has been possible to breed 

 millions of insect larvae and to determine accurately the 

 incidence of some of the important causes of mortality, and 

 it is not unlikely that further developments of similar methods 

 may eventually give us a reasonably complete picture of the 

 death-rate in a few species. 



We prefer to take this optimistic view because there are 

 grave difficulties in the employment of indirect evidence. 

 The bulk of the latter aims at showing that certain structures 

 or habits are ' useful.' This does not prove that they are 

 actually, on the balance, of survival value to their possessors. 

 To do this we should have to compare the death-rates of forms 

 with and without the structure or habit in question. But this 

 comparison involves the study of the direct evidence for the 

 selection theory. 



Again, it is usually stated that the relations of any animal 

 to its environment are so complicated that we can never hope 

 fully to demonstrate the action of Natural Selection, and in 

 particular can never show it is not operative in a given case. 

 This argument is commonly brought forward to explain the 

 apparently non-adaptive specific characters. But the appeal 

 to ignorance is two-edged and cuts both ways, and cannot be 

 used to turn apparently unfavourable instances to advantage. 

 That is too much like a marksman who, seeing his birds flying 

 away, says that for all he knows they may belong to a variety 

 resistant to shot. 



When Darwin wrote, it was very important to convince 



