GENETICS 83 



former case, that essentials must be equally im- 

 portant, although they may differ in the nature 

 of their importance. The determinative powers of 

 the genes have concentrated attention upon the 

 chromosomes, but we have no evidence that they 

 can exist independent of the cytoplasm. What then, 

 may be the capacity of the cytoplasm for a distinc- 

 tive role in the determination of the organism .^^ 



Numerous attempts have been made to estab- 

 lish the theory that some of the cytoplasmic inclu- 

 sions, such as mitochondria, have functions in 

 heredity comparable to those of the chromosomes, 

 but without success. That such inclusions not 

 only occur, but in some cases divide and are handed 

 down to daughter cells, is well known, particularly 

 in plant cells where the chloroplastids are per- 

 petuated in this way. Morgan says of such bodies 

 that "they are inherited but not with the precision 

 of the genes. There has never been any serious 

 attempt to ignore this kind of heredity, though it 

 has seemed to me desirable to keep it in a chapter 

 by itself. . . . except for the rare cases of plastid 

 inheritance, the inheritance of all known charac- 

 ters can be sufficiently accounted for by the pres- 

 ence of genes in the chromosomes. In a word the 

 cytoplasm may be ignored genetically." ^^ He calls 

 attention also to the evidence furnished by recipro- 

 cal crosses. The hybrids so produced should differ 



" Am. Nat., Vol. LX. p. 491, 1926. 



