XV] Chromosomes and Heredity 271 



Much that is known of chromosomes seems inconsistent 

 with the vie^ that they are the sole effective instruments 

 Jn heredity. 



Wi5!l21^^^^?^i^SJ^P ^-^^fi^^t^ opinion on this question, 

 I venture to state what seem to me formidable difficulties 

 in the way of this expectation. If the chromosomes were 

 directly responsible as chief agents in the production of 

 jthe physical characteristics, surely we should expect to find 

 some degree of correspondence between the differences 

 ^distinguishing the types, and the visible differences of 

 number or shape distinguishing the chromosomes. So far 

 ^s I-xan learn, no indication whatever of such a corre- 

 spondence h_asjeyer been found. Besides this, although no t / 

 very thorough investigation of the chromosomes of somatic ''^ 

 structures has yet been made on an extensive scale, I 

 believe that consistent cytological distinctions between the 

 nuclei of the various tissues of the same dody Jiaive no t been 

 detected. If chromosomes were the chief governors of 

 structure, surely we should find great differences between \^ / 

 the chromosomes of the various epithelia, which differ 

 greatly in th eir _ structure, and properties. As these cyto- 

 jogicai diffe mices have not been found consistently there, 

 the prospec t orsuccessfully tracin g the m amo ng the specific 

 types does not Jpok very hopeful. 



Again, no correspondence between the chromosome 

 numbers aiiE complexity of structure has ever been asserted 

 to~exist. Low forms may have many ; highly complex types_ 

 ISay^teveibT^r — — - 



Then, on the contrary, very closely allied types may 

 show great differences in these respects. As is well known, 

 Rosenberg has shown that one species of Drosera has 20, 

 while another has 10. Again, Miss Lutz, and, independ- 

 ently, Gates, have found remarkable diversities in Oenothera, 

 especially \\vaX gigas has 28, while tata has 14^. Obviously 

 this doubling means something definite, but it is not sug- 

 gestive of the determination of specific difference. 



In Aphis Miss Stevens, on the other hand, has shown 

 how wide a diversity may be presented by the chromosomes 

 of forms so alike as to have passed for one species. These 



* Important evidence as to variations in chromosome numbers has 

 been published by R. R. Gates, Botanical Gazette^ July, 1908. 



