568 Species According to the Theory of Mutation. 



a much more complicated structure and have arisen in 

 a historical way. They cannot be isolated and then sub- 

 jected to experiment like chemical bodies; we can only 

 investigate them by studying the relation of closely allied 

 species and varieties, i. e., forms in which a definite unit, 

 or several of them, are present in one plant and absent in 

 the other. For this reason our investigations are, for 

 the present at any rate, confined to the units which have 

 arisen most recently. 



But even as it is the business of comparative science 

 in general, first, to apply the conclusions derived directly 

 from the facts, to cases that have not been themselves 

 observed and then to extend them gradually further and 

 further, it is our duty and our riglit to test the applica- 

 bility of our conclusions as thoroughly and as widely as 

 possible. 



Therefore we have now to face the question whether 

 the theory of the origin of species by mutation and the 

 theory that hereditary characters are composed of ele- 

 mentary units are in harmony with the theoretical concep- 

 tions to which systematic science on the one hand and 

 embryology on the other have given rise. If it can be 

 shown that the mutation theory satisfies the demands of 

 these sciences better than the present form of the theory 

 of selection, its justification as a theory of the nature 

 of inheritance will, in my opinion, be placed on a sure 

 foundation. 



For this reason I shall devote the last part of tin's 

 work to general considerations of this kind. In doing 

 so I leave the safe ground of facts and venture into a 

 region in which I can no longer mainly depend on my 

 own experience. But experimental inquiry must derive 

 its problems from tin's more general aspect of the ques- 



