The Explanation of . Uiapfatiotis. 609 



A strong argument for my view was \)u{ forward by 

 Rosa and Cattaneo.^ According to these authors the 

 extinction of large groups of species proves that the 

 varial)ihty resident in them was powerless to ada])t them 

 to the clianging conditions of life; and fnjm this cun- 

 clusion they infer that the ordinary variability, as it is 

 alicays manifested, is not sufficient for this ])urpose. Ob- 

 viously some other process is necessary. 



][. Fluctuating variability is linear; it oscillates only 

 in a plus and 2.niinns direction, whilst adaptations demand 

 a variability which will produce variations in all direc- 

 tions.- On this point also I have expressed my opinion 

 in the first volume (p. 118). It constitutes, in my opin- 

 ion, one of the strongest objections to the prevailing 

 view ; and it also shows more clearly than anything else 

 how far Darwin's adherents have departed from the 

 views actually expressed by him. To Darwin's mind 

 the essential point was that the struggle for existence 

 should have to select from material supplied by an in- 

 determinate variability. Natural selection is a sieve. It 

 creates nothing, as is so often assumed; it only sifts. 

 It retains only what variability puts into the sieve. 

 Whence the material comes that is put into it, should l:>e 

 kept separate from the theory of its selection. How the 

 struggle for existence sifts is one question; how that 

 which is sifted arose is another. In both respects, Dar- 

 win's original view is still the best, but the point at issue 

 has been often obscured by later writers. The meshes 

 of the sieve are not such as to separate only the very best ; 

 on the contrary natural selection only throws out some 

 part of the individuals, and amongst them the worst, i. c.. 



^ See below in § 12. 



^GusTAV Wolff. 7^(V" gcgcnwaytigc Sland dcs Darwtnisinus, i.v/). 



