RESULTS OF CROSSING TWO HEMIPTEEOUS SPECIES. 475^ 



Ijinhage. 



To the cytologist, linkage of characters in inheritance is of special interest 

 because it is claimed that it affords ih.Q most trustworthy evidence that the 

 factors determining linked characters are located in the same chromosome, 

 and further it is claimed that this chromosome can be identified. Wilson has 

 recently expressed this view clearly in bis Croonian Lecture (^14). After 

 giving a brief summary of the work of Morgan and his pupils on linked 

 characters in Drosopliila, he adds : — 



"This at once suggests that the units of each group (or corresponding 

 things on which they depend) are borne by a particular chromosome which 

 constitutes their common vehicle of transmission, and that to this fact is due 

 their cohesion or linkage in heredity. Conversely, the several groups are 

 independent of one another, because of the independence of the chromosomes 

 which bear them." (-Pfige 34-4.) 



If, as Wilson says, independence in the transmission of characters is 

 due to independence of the chromosomes which bear them, the evidence 

 obtained from our cross-breeding experiments would indicate that the factors 

 determining the transmission of the intromittent organ are not only not 

 carried by a single pair of chromosomes ; but on the above hypothesis 

 it would seem difficult to confine them to the 14 chromosomes, for among 

 the 190 F2 hybrids there ai'e 69 different lengths of the intromittent 

 organ, and if size-variations are due to multiple unit factors which are- 

 transmitted MS independent units, these 69 variations would seem to demand 

 an explanation from those who believe that ''unit factors^' are located in the 

 chromosomes. Further, none of these Q*i!i variations in the F2 hybrids is 

 consistently linked with any of the variations of the genital spot. We 

 might reduce the number of independent variations of the intromittent 

 organ and the genital spot by consigning most of them to the convenient 

 class called " non-inheritable fluctuations ", but this rather arbitrary process 

 must be carried far, if the remaining " unit factors " are to be consigned 

 to a single pair of chromosomes. 



The " cross-over hypothesis " , which was offered to explain unexpected 

 results in the transmission of characters assumed to be carried by special 

 chromosomes, might be used to excuse non-linkage in these extreme cases ;: 

 but we cannot believe that it wonld be adequate to convince the unprejudiced 

 investigator that the factors decei'mining quantitative variations in the 

 intromittent organ and genital spot are carried and distributed by the 

 chromosomes. Even if we arbitrarily consign the determining factors to 

 special positions in the chromosomes and dictate their subsequent method 

 of division, it does not seem possible to adjust the facts with the view that 

 linkage and non-linkage may have their explanation in chromosome- 

 distribution of the factors. 



38* 



