194 THE SECOND-CHROMOSOME GROUP 



is probably no double crossing-over whatever within it. The second 

 difficulty was met by Plough in his similar studies on the temperature 

 variation of linkage, by transferring his parents every 2 days instead of 

 every 10 (Plough, 1917). As one of his control experiments Plough 

 ran a black purple curved back-cross of 13 pairs, transferring each 

 pair to a fresh culture-tube every 2 days as long as the female lived 

 (table 14, Plough, 1917). The plotted curve of the percentages of 

 crossing-over between black and purple shows an initial high value 

 (8 per cent) which during the first 9 days falls rapidly at first and then 

 more slowly to a low value (5 per cent), which is maintained with little 

 change to about the sixteenth day. A sharp rise then sets in which 

 reaches its maximum (8 per cent) at about the twenty-first day. The 

 succeeding fall is again slow, reaching its minimum (3.5 per cent) at 

 about the thirtieth day. Beyond this point the curve again rises 

 slightly; but the data were too few to be significant beyond about the 

 twenty-fifth day. While there was some variation in the amount and 

 rapidity of these changes in the various individual curves, all showed 

 the same typical rhythm, which must be the expression of fundamental 

 physiological changes in the development of the female. It seems 

 possible and probable that these successive falls and rises are not effects 

 of a single continuously varying physiological process, but are rather 

 to be explained as separate phenomena caused by the lapse of certain 

 conditions and the subsequent onset of new causes. These changes 

 may therefore be really discontinuous and the rhythmic curve only a 

 succession of independent but overlapping variations. 



The most interesting feature of the age-variation is the bearing it 

 has on the problem of double crossing-over and the underlying problems 

 of the nature of crossing-over. The more that consideration has been 

 given to this problem of double crossing-over in relation to chromosome 

 and to map-distances, the more involved it has appeared, so that no 

 evidence upon these points can be neglected. The special value of 

 such cases as that of age-variation is that they enable one to compare 

 two different conditions but with the elimination of one important 

 variable; for the actual chromosome-distance between such genes as 

 black and purple is maintained constant, so that any variations that 

 occur in map-distance, coincidence, etc., must be due to variations in 

 one or more of the other factors. This relation was discussed in con- 

 nection with the original two-brood black purple curved cross (Bridges, 

 1915), and it was pointed out that the rise in coincidence concomitant 

 with the fall in crossing over meant that the internode length had 

 changed that the two cross-overs of a double no longer included 

 the same average length of chromosome, but a longer length. The 



8' 



coincidences of the case support this view, but in neither 



Pr S, 



