704 Mutations 



ing-primrose as one unit-stride section in the 

 great genealogic tree, this period includes two 

 nearly related, but not identical changes. One 

 is the production of new specific characters in 

 the latent condition, and the other is the bring- 

 ing of them to light and putting them into active 

 existence. These two main factors are conse- 

 quently to be assumed in all hypothetic concep- 

 tions of iDrevious mutative periods. 



Are all mutations to be considered as limited 

 to such periods? Of course not. Stray muta- 

 tions may occur as well. Our knowledge con- 

 cerning this point is inadequate for any definite 

 statement. Swarms of variable species are 

 easily recognized, if the remnants are not too 

 few. But if only one or two new species have 

 survived, how can we tell whether they have 

 originated alone or together with others. This 

 difficulty is still more pronounced in regard to 

 paleontologic facts, as the remains of geologic 

 swarms are often found, but the absence of 

 numerous mutations can hardly be proved in 

 anv case. 



I have more than once found occasion to lay 

 stress on the importance of a distinction be- 

 tween progressive and retrograde mutations in 

 previous lectures. All improvement is, of 

 course, by the first of these modes of evolution, 

 but apparent losses of organs or qualities are 



