88 EVIDENCES AND LIMITATIONS 



discontinuous variation, taken advantage of by breeders. 

 If he is right, the phenomena of artificial selection 

 afford no support to Darwin's belief that natural selec- 

 tion is the chief factor in species forming; but they do 

 appear to furnish direct evidence of natural evolution.^ 

 Whether he is right or not remains for further investi- 

 gation finally to determine; but in any case the fact 

 that new species have been actually observed to origi- 

 nate by natural evolution is clear. No doubt the in- 

 stances observed are too narrow in range to afford a 

 basis for conclusive generalization, but they are in line 

 with the evolutionary hypothesis, and are not offset 

 by contrary evidence. 

 ^ My summary of the evidence by which the theory 



of natural evolution is supported has necessarily been 

 brief and inadequate. But I trust that it will enable 

 those among you who have not previously studied the 

 subject to perceive that the evolutionary hypothesis, 

 considered as a description of the physical aspects of 

 species forming, holds the field. That theory is in 

 harmony with the facts which have hitherto been ob- 

 served, and works well as a means of interpreting phe- 

 nomena which are otherwise unintelligible. It has no 

 rival in scientific thought. It ought, therefore, to be 

 accepted as the best physical description of the origin 

 of species that is now available.^ 



1 On the observations of de Vries and Morgan, see T. H. Mor- 

 gan, Evolution and Adaptation; V. L. Kellogg, Darwinism To-day, 

 pp. 341-348 (cf. pp. 362-373); R. H. Lock, Recent Progress, pp. 128- 

 147. Cf. pp. 69-72, above. 



2 This does not mean a dogmatic assertion that natural evolution 



