HETEROPLOIDS 187 



the trivalent chromosomes differs from the other two by 

 a duplicated half. Hence the secondary has a different 

 gene combination from the primary. 



Kuwada reports 20 chromosomes (n=10) for corn 

 (Zea mays), but certain sugar corns were found to have 

 21, 22, and even 23 or 24 chromosomes. Kuwada suggests 

 that corn is a hybrid, one of whose parents was the Mexi- 

 can teosinte (Euchlaena). One of the corn chromosomes 

 that is longer than its mate was derived from teosinte, 

 he thinks, and its mate from some unknown species. The 

 longer one sometimes breaks into two pieces, which ac- 

 counts for the additional chromosomes found in sugar 

 corns. If this interpretation is verified (it has recently 

 been questioned), these 21, 22, and 23 chromosome types 

 are not strictly trisomic. 



De Vries' conclusions relating to the extra chromosome 

 types of Oenothera Lamarckiana had an important bear- 

 ing on his interpretation of the origin of progressive 

 mutation, hence on his interpretation of the relation of 

 mutation to evolution. The numerous small changes in the 

 characters of the individual frequently observed in triso- 

 mic types fulfill de Vries' early definition as to what 

 constitutes an elementary species, causing at a stroke, 

 as it were, the appearance of two elementary species. 



It should be observed that when a mutational effect is 

 produced by the addition of a whole chromosome the re- 

 sult involves, so far as the germ material is concerned, 

 an enormous alteration in the actual number of the he- 

 reditary units. This change is scarcely compatible with 

 the comparison to a change in a single chemical molecule. 

 Only by treating the chromosomes as a unit could such a 

 comparison have any weight. The constitution of the 

 chromosomes, from the viewpoint of their genes, is hardly 

 consistent with such a comparison. 



The chief interest in these heteroploids, as I interpret 



