V. THE ORIGIN OF SPECIES BY MUTATION. 



§ 21. SPECIES, SUBSPECIES AND VARIETIES. 



We saw in the second chapter that species cannot 

 have originated by the natural selection of the extreme 

 variants afforded by fluctuating variability. 



We have therefore now to show that the observations 

 which have been made on this subject can be simply and 

 completely explained on the hypothesis of sudden changes. 

 When such transformations occur among cultivated plants 

 — and they often do — they are called spontaneous or, 

 as Darwin called them, single variations : moreover they 

 are almost always inherited, if not in their entirety, at 

 any rate to a very considerable extent. 



We may express therefore the essence of the Muta- 

 tion theory in the words : "Species have arisen after the 

 manner of so-called spontaneous variations.'' And in 

 our critical survey of the facts we therefore have to con- 

 sider how far the information at our disposal justifies 

 this view. 



In order to be qualified to discuss this question we 

 must first of all make quite sure what we understand by 

 the term "species" and, more important still, we must 

 form a clear idea as to which forms we are going to re- 

 gard as the units of the natural system. For it is only 

 in the case of the real units of the system that we can 



