68 THE THEORY OF THE GENE 



units are bound up in groups, and, in related species, the 

 same units and groups of units recur. Transitions, such 

 as seen in the outer forms of animals and plants, no more 

 exist between the units than between the molecules of the 

 chemist. 



''Species are not continuously connected, but arise 

 through sudden changes or steps. Each new unit added 

 to those already present forms a step, and separates the 

 new type as an independent species from the species from 

 which it arises. The new species is 'presto change,' there. 

 It arises without visible preparation and without transi- 

 tions." 



It may appear from this statement that a mutation 

 that produces a new elementary species is due to the 

 sudden appearance or creation of a new element — a new 

 gene. Put in another way, we witness at mutation the 

 birth of a new gene or at least its activation. The number 

 of active genes in the world has been increased by one. 



De Vries has further elaborated his views on mutation 

 in the concluding chapters of The Mutation Theory and 

 in his later lectures on ' ' Species and Varieties. ' ' He recog- 

 nizes two processes, one the addition of a new element 

 that gives rise to a new species ; and the other, the inacti- 

 vation of a gene already present. It is the second view 

 that interests us at present, because, except for the man- 

 ner of expression, it is essentially the view that is today 

 sometimes said to be the way in which the new types in 

 our cultures arise — through the loss of a gene. De Vries 

 himself, in fact, places in this category all the commonly 

 observed cases of loss mutations without respect to their 

 dominance or recessiveness, implying, however, that they 

 are recessive because their gene has become inactive. 

 Mendelian results, he thinks, belong solely to this second 

 category, because of the existence of contrasting pairs of 

 genes — the active one and its inactive mate. These segre- 



