128 THE PROBLEMS OF EVOLUTION 



two treasures of the geneticists, Oenothera and 

 Drosophila, are the outstanding examples. 



The objection has frequently been advanced 

 that the species which give rise so freely to muta- 

 tions are merely natural complex hybrids from 

 which homozygous combinations occasionally seg- 

 regate. Such a condition is conceivable, but if all 

 mutants were homozygous recessives, then reverse 

 mutations could not occur. Reverse mutations 

 actually do occur, however, both in Oenothera and 

 in Drosophila. Moreover some mutant characters 

 are dominant over the natural characters which 

 preceded them; if they were merely segregated in 

 homozygous dominants any visible difference must 

 have been evident in previous generations. And 

 lastly some mutations are characters which did 

 not appear at all in previous generations; they are 

 apparently new characters arising from some 

 chromosomal modification. In short, the evidence 

 is wholly against the view that species which pro- 

 duce mutations are hybrids in the natural state; 

 such conditions may occur, but mutations un- 

 doubtedly occur also as actual departures from the 

 previous condition of the heritage. 



Granting the occurrence of mutations as definite 

 modifications of an existing heritage, two possible 

 evaluations are open to us. De Vries himself pro- 

 vided the interpretation that they are the direct 

 source of new species, and named his Oenothera 



