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THE AMEBIC AN NATURALIST [Vol. L 



vious rise. In addition, the curve should, on this explana- 

 tion, rise high above the 100 per cent, level at its modal 

 point, whereas it is evident that, so far as the significant 

 figures go, it does not rise much above 100 per cent, at any 

 point. It would be premature, however, to generalize fur- 

 ther on these results. 



The curve for group II will not be presented until 

 greater numbers of flies have been recorded. It may be 

 stated, however, that this curve too shows the phenomenon 

 of interference, although, since the factors are not so close 

 together, the crossing-over for rather small distances can- 

 not so well be followed. 



The great variability possible in the distance between 

 two points of crossing-over is shown not only in the above 

 curves, but may be graphically illustrated from a single 

 case. This fly was the triple cross-over in the first chromo- 

 some, which has already been mentioned. Its mother was 

 one of the tested females of the count, whose composition 

 proved to have been j^l ^ msr ? an( j n itself was a male 

 with the factors yrB r . Crossing-over, therefore, must have 

 taken place between y and w, s and r, and r and f . The 

 minimum possible distance between the first two points of 

 crossing-over is 42, the maximum distance between the 

 second two is 14. The latter is the smallest distance ever 

 observed between two points of crossing-over. It may 

 here be mentioned that it will be of great interest, when 

 more extensive figures are obtained, to see whether in the 

 second chromosome the same coincidence holds between 

 crossings-over on opposite sides of the middle point as 

 between crossings-over an equal distance apart, but on the 

 same side. The bend of the chromosomes in the middle, or 

 some other structural difference here due to the attach- 

 ment of the spindle fiber at this point, might cause the re- 

 sults to be different in the above two cases. 



Incidentally, the results demonstrate another point, 

 lying in a somewhat different field of genetics. By fol- 

 lowing the method of keeping stocks constantly in heter- 



