MUTATION THEORY OF PROFESSOR DK VRIES. 633 



not occur flowingly. but in steps, without transition, and it occurs less 

 frequently than do the common variations, which are continuously and 

 constantly at hand. The contrast between the two kinds at once 

 appears if one considers the proposition that the attributes of organ- 

 isms are built up of fixed and sharply defined units. These units com- 

 bine in groups, and in the kindred of species the same units and 

 gi'oups are reproduced. The origination of a new unit signities a 

 nuitation. Every addition of a unit to a group constitutes a step, 

 originates a new group, and separates the new form sharply and fully 

 as an individual species from the one out of which it has been pi'o- 

 duced. The new species is at once such, and originates from the 

 former species without apparent preparation and without gradation. 

 Each attribute, of course, arises from one previously present, not by 

 their normal variation, but by one small yet sudden change. Pro- 

 vi.9ionally. one may compare these changes, but only in the simplest 

 manner, with chemical substitution. 



''In the tirst section of this book the nuitation theor}' is contrasted 

 with the selection theory. The latter assumes the common variability 

 to be the starting point of the origin of a new species, but according 

 to the mutation theory the two processes are quite independent of each 

 other. Common variability, as I hope to show, does not lead, by even 

 the sharpest persistent selection, to any real transgression of the limits 

 of species, much less to the origin of new and constant attributes." 



It is in the first section of Volume I that the author discusses at 

 length selection and mutation, mutability and variability, all the vari- 

 ous theories of evolution that have been proposed, the effects of cli- 

 mate and of horticultural breeding, the limitations and characteristics 

 of species and varieties, and other pertinent subjects. He concludes 

 that section of the volume with the following summary" concerning the 

 nature of his theor}" 



"(1) The doctrine of morphological and historical descent deals Avith 

 the origin of the Linnean, or collective species, the genera, families, 

 and higher groups. The doctrine of experimental descent deals with 

 the origin of elementar}" species, or, more strictly speaking, with the 

 origin of specific characters. 



"(2) 'The true danger reef of the Darwinian theory is the transition 

 from artificial breeding selection to natural selection.' (Paul Janet.) 

 This reef can only be avoided when one recognizes the improvement of 

 races and the origination of new forms as two entirely different occur- 

 rences, only apparently' passing into each other. Eor Darwin, the}' 

 stand side by side, the one in no way excluding the other, although, as 

 a rule, he has not sharply differentiated them. 



" (3) 'No two individuals of any planting are entirely alike.' This 

 well-known proposition is to l)c confined to the province of real fluctu- 

 ating variability. It stands in no relation to the doctrine of descent, 

 if one accepts the nuitation theory. 



"(4) 'Species have originated l)v natural selection in the struggle 

 for existence.' But this statement needs explanation. The struggle 

 for existence — that is, competition for existence — embraces two entirely 

 different points. Whenever the contest occurs between individuals of 

 one and the same elementary species, it also occurs bet\ve(Mi the differ- 

 ent species as such. The first-mentioned contest pertains to the 



