POST-DARWINIAN VIEWS 71 



cause of improvement. ... It is only a sieve, which 

 decides which is to Hve, and what is to die. ... It is 

 the sieve that keeps evolution on the main line, kilHng 

 all or nearly all that try to go in other directions. By 

 this means natural selection is the one directing cause 

 of the broad hues of evolution." ^ In brief, the muta- 

 tions theory makes natural selection begin its work 

 with species already formed, and restricts its opera- 

 tion to an ehmination of unfit species and an exclusive 

 preservation of such as are fitted to survive. 



The mutations theory is free from some of the most 

 troublesome difficulties attendant upon the view of 

 Darwin, that new species arise from natural selection 

 and an accumulation of continuous variations. More- 

 over, de Vries has been able to furnish direct evidence, 

 for he has observed actual instances of sudden origin 

 of new species by mutation of the discontinuous kind. 

 It has been objected that these instances, while they 

 estabHsh the fact that species do at times thus origi- 

 nate, are altogether too few to warrant such a general- 

 ization as is represented by the mutations theory. It 

 has been answered that, as the origin of new species 

 is not an every-day event, the instances observed must 

 necessarily be somewhat Hmited in number, and that 

 further observation will probably afford additional 

 evidence, now that investigators are looking in the 

 right direction. Furthermore, it is urged that, if the 



1 Species and Varieties, Their Origin by Mutation (ed. by Mac- 

 Dougal, 1905), p. 6. On the mutations theory, see R. H. Lock, op. 

 cit., ch. v; V. L. Kellogg, op. cit., ch. xi. The facts upon which the 

 theory is chiefly based are given below, pp. 87-88. 



