xv] Chromosomes and Heredity 271 



Much that is known of chromosomes seems inconsistent 

 with the view that they are the sole effective instruments 

 in heredity. 



Without presuming to a definite 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 

 the 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 

 as I can learn, no indication whatever of such a corre- 

 spondence has ever been found. Besides this, although no 

 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 body have not 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 their structure and properties. As these cyto- 

 logical differences have not been found consistently there, 

 the prospect of successfully tracing them among the specific 

 types does not look very hopeful. 



Again, no correspondence between the chromosome 

 numbers and complexity of structure has ever been asserted 

 to exist. Low forms may have many ; highly complex types 

 may have few. 



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 Oenoihera, 

 especially that gigas has 28, while lata 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. Qz.tes, Botanical Gazette, July, 1908. 



