48 DARWIN AND NATURAL SELECTION 



of Botany in Amsterdam and the author of the most 

 recent work on Mutation : " Natural Selection acts 

 as a sieve ; it does not single out the best variations, 

 but it simply destroys the larger number of those 

 who are, from some cause or another, unfit for 

 their present environment. In this way it keeps 

 the strains up to the required standard, and, in 

 special circumstances, may even improve them."* 

 So far, then, for the main outlines of the theory. 

 The quotations given abundantly prove at least 

 one point, namely, that scientific opinion to-day 

 is anything but unanimous as to what natural 

 selection can do, or even whether it can do any- 

 thing at all. It remains to examine certain sub- 

 sidiary problems arising out of the doctrine, and 

 here again I shall take pains to point out what 

 recent writers have said on each problem as it 

 arises. First of all, then, it is clear that if natural 

 selection is a sieve, it must have something to sift 

 that is to say, there must be such things as vari- 

 ations to be operated upon. We need not linger 

 over this point, for no one doubts that variations 

 do occur, though, as we shall shortly see, there is 

 considerable difference of opinion as to the kind 

 of variations which really count in connexion with 

 natural selection. The real question is how these 

 variations come about ; that is the question of 

 questions in Biology in fact, as Samuel Butler 

 most acutely observed, " To me it seems that the 

 6 Origin of Variation,' whatever it is, is the only 

 true ' Origin of Species,' "t 



* Darwin and Modern Science, p. 70. 

 t Life and Habit, p. 263 (1910 ed.). 



