470 THE AMERICAN NATURALIST [Vol. XLIX 



males arise. The somatic condition of the females may 

 therefore be termed XX and that of the males XY. If 

 both reduced normally at any time, ordinary fertilization 

 might be expected to give both males and females. But 

 the spermatocytes without X degenerate, leaving only one 

 type of functional spermatozoa, which produces females. 

 Thus actual causal connection between the X chromosome 

 and sex determination appears to have been demonstrated. 



These are the main cytological arguments in favor of 

 the chromosome view of heredity that seem to me to be 

 insuperable. There are minor arguments both pro and 

 con, which, as I said in the beginning, we have not space 

 to consider. Instead it seems more profitable to show 

 how Mendelian results interlock with those from cytology 

 like the parts of a jig-saw puzzle. 



Chromosomes and Mendelian Inheritance 

 The principal phenomena of Mendelian inheritance are: 

 (1) characters that breed true; (2) uniformity of the 

 population of the first hybrid generation in particular 

 traits in which homozygous parents differed; (3) inde- 

 pendent segregation of certain character determiners; 

 (4) recombination of certain characters; (5) perfect 

 coupling between certain characters; and (6) partial 

 coupling between certain characters. Let us see how 

 plausibly one can picture the mechanism through which 

 such phenomena may result without imputing to the 

 chromosomes any behavior that is not known to occur. 

 To do this simply let the imagination portray a plant spe- 

 cies having four chromosomes, each chromosome having 

 three character determinants that can be followed through 

 the breeding results that are obtained. 



Our figures represent the immature germ cells of the 

 plant just previous to the reduction division. Fig. 1 

 shows the germ mother cell with a duplicate set of heredi- 

 tary determinants. The mature germ cells are exactly 

 alike, therefore the plant breeds true to the characters 

 concerned. 



