THE MUTATION THEORY 



517 



species. Some one must find out whether or not the un- 

 paired condition occurs in hybrids whose parents do not 

 show it. Strange as it may seem, after all the discus- 

 sion of hybridity as a possible cause of mutability, no one 

 has yet shown, or tried to show, that mutability occurs 

 in any hybrid between non-mutable parent species. This 

 would seem to be one of the most crucial experiments that 

 could be performed, and one of the easiest. It would be a 

 very attractive problem to attempt to produce the un- 

 paired chromosome condition by hybridization and then 

 to prove it definitely correlated with the particular types 

 of mutability which are characterized b\ disturbances of 

 the chromosome mechanism. 



Even if it were possible in the time at my disposal to 

 review the evidence that the chromosomes provide the 

 mechanism of inheritance, it would hardly be necessary 

 to do so. The brilliant work of Morgan and his students 

 on the association in Drosophila of groups of characters 

 with definite chromosomes is well known to every one. In 

 (Enothera the investigations of Gates, Lutz and others 

 have shown a connection between chromosome alterations 

 and the characters of certain mutations so obvious that 

 it can not reasonably be disregarded. 



There are still a few geneticists, however, who believe 

 that the chromosome cycle has no fundamental signifi- 

 cance in connection with Mendelian phenomena. Heri- 

 bert-Nilsson would ascribe as much weight to a change 

 from flat to crinkled leaves, to choose an example at 

 random, as he would to a change from the 2x to the 4x 

 chromosome number. Such an attitude is forced upon 

 one who attempts to explain all mutations as Mendelian 

 segregates, as this author does. He has made much of a 

 ease, observed by Geerts and Stomps, in which the chro- 

 mosome number of a hybrid between (Enothera gigas and 

 CE. Lamarckiana became reduced from 21 to 14, probably 

 through the agency of an irregular reduction division, 

 without the loss of the gigas characters. This case is 

 cited to prove that the characters of this mutation do not 

 depend upon the supernumerary chromosomes. Nothing 



