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THE AMERICAN NATURALIST [Vol. XLIII 



have as a simple Mendelian character. If it be due to differences 

 in two sets, it will behave as a compound character of two factors, 



All known Mendelian phenomena may thus be explained as 

 due to differences in the chemical constitution of the chromo- 

 somes iii different ".roups. It is thus seen that Mendelian 

 phenomena lend no support to the theory that each hereditary 

 character is represented in the germ plasm by a separate entity. 



The question as to the nature of the chromosome differences 

 which are thus seen to be able to account for the phenomena 

 first interpreted by Mendel will be considered at another time. 

 The differences between the metabolic activities of homologous 

 chromosomes here assumed may be due to differences in the 

 relative amounts of given substances in the chromosomes con- 

 cerned, or they may be due to differences existing in different 

 nt/ions of the chromosomes. In the present state of our knowl- 

 edge of the chromosomes we are not ready for any theory on 

 this point. Should Shull or Baur succeed in getting into a single 

 individual more independent (neither correlated nor allelo- 

 morphic) dominant characters than there are chromosome pairs, 

 then we shall at least know that the chromosome as an individual 

 structure is not responsible for Mendelian characters. This is 

 the one question which must be settled before Mendelian theory 

 can make further progress. 



Much recent work has been done which bears on the relation 

 between chromosomes and hereditary characters. 



First of these should be mentioned the important contribution 

 made by Professor E. B. Wilson, published in Science, January 

 8, 1909. This paper is so accessible that it is unnecessary here 

 to review it in full. Suffice it to say that Professor Wilson and 

 his students have demonstrated an important relation between 

 sex and certain chromosomes and chromosome groups. In gen- 

 eral, the cells of the species studied contain an "X-element" 

 which in some species consists of one chromosome, in others of 

 two, in others three, and in one species of four chromosomes, but 

 which acts as a unit in the reduction division; i. e., all the chro- 

 mosomes of the "element" pass to the same pole. In all the spe- 

 cies studied, the cells of the female contain two of these X-ele- 

 ments, while those of the male contain but one. The males of 

 some of the species contain no homolog (synaptic mate) for this 

 element, but others contain an element which Wilson calls the 

 Y-element, with which the X-element pairs in the reduction divi- 

 sion. In every case the Y-element, when present, consists of a 



