PT. II, CH. XXI] THE MUTATION THEORY 223 



altered, and these alterations are now generally called mutations. 

 The theory assumes that these only are connected with the origin 

 of species and varieties. 



Darwin recognised both mutation and fluctuation as steps in 

 the general process of evolution. For this assertion he mainly 

 relied on his studies of the variation of animals and plants under 

 domestication, since organisms in the wild condition did not, at 

 his time, afford a sufficient basis for controlling his conception. 

 He assumed mutations to be of subordinate significance, explain- 

 ing the main lines of the evolutionary process on the assumption 

 of individual or gradual variation. This variation he had shown 

 to occur everywhere, but as to its capability of achieving lasting 

 changes, he had no facts at hand to give a definite proof. 



In my book on the Mutation Theory I have given an elaborate 

 "Review of the Facts," especially on the botanical side, in order 

 to show that fluctuating variability does not lead to durable 

 changes in the hereditary composition of a type. Wherever such 

 changes occur they may be shown to be historically, or at least 

 probably, due to saltations. These critical considerations led to 

 the proof that the conception of mutations was in full harmony 

 with our knowledge of the variability of plants, as it occurs 

 everywhere in nature as well as in horticultural and agricultural 

 breeding. 



The mutation theory is intended to be a support and a corol- 

 lary of the selection theory of Darwin. There can be no doubt 

 that Darwin correctly set forth the essential steps in the evo- 

 lutionary process and that changes in his views mostly relate 

 to those minor points, for which, at his time, the material of 

 facts was not adequate to a correct decision. The mutation theory 

 claims to remove many of the difficulties, inherent to the Dar- 

 winian doctrine, as e.g. the general occurrence of useless charac- 

 ters and the impossibility of explaining the first beginning of a 

 selection on the ground of its usefulness. 



In order to become generally accepted this theory has to be 

 considered from two main points of \'iew. The contention that 

 species and varieties originate by mutation is essentially experi- 

 mental in its nature. But the thesis that they cannot be shown 

 to have ever originated in another way has to be studied in the 

 field of systematic botany and zoology, and partly in that of 

 palaeontology. Mutations were well known to Darwin to occur 

 from time to time, and of late numerous observations of special 

 cases in animals and plants have been published. A list of them 



