72 THE EVOLUTIONARY THEORY 



number of instances of mutation which have been 

 observed is small, no indisputable instances whatever 

 have been perceived of the origin of new species by 

 the selection of accumulating continuous variations. 

 The evidence by which Darwin's theory is thought to 

 be estabHshed is wholly indirect — being either con- 

 jectural or, at least, circumstantial. The battle has 

 not been entirely fought through, and it is too early 

 at present to come to final conclusions as to the out- 

 come. But the new theory has brought the difficulties 

 which attend neo-Darwinism into bold rehef, and we 

 are not Hkely to see any permanent return to pure 

 Darwinism. 



I have ignored other theories of species forming, of 

 which several have appeared during the past two or 

 three decades,^ because none of them have seemed to 

 gain the attention of scientists that has been paid 

 to Weismannism and the mutations theory. But I 

 ought not to omit mention of the fact that there is an 

 increasing reahzation among scientists of the part played 

 in evolution by what are called "unknown factors." 

 The problem as to the causes of variation, and as to 

 the laws which determine their direction and Hmits, 

 has become more and more pressing. The fact that 

 natural selection does not positively cause variation, 

 but merely eliminates unsuitable results thereof, is 

 now well established. IVIany scientists have escaped 

 the hampering influence of naturahsm, and perceive 



1 A general account of them, with numerous references, is given 

 by V. L. Kellogg, op. cit., chh. viii-x. 



