112 EDWIN CARLETON MACDOWELL 



called to the fact that this line of single families has not been 

 chosen with any regard to the grades of the flies. This is the 

 only line that can be continuously followed to the end of the 

 experiment. 



Means 



Before any interpretation of the means of the high selected 

 race can be attempted, certain facts must be made clear. In 

 generations 24, 25 and 26 there hatched only one family each. 

 In generation 26 unfavorable conditions nearly caused the loss 

 of the whole race. In order to continue the experiment parents 

 with fewer bristles than the standard for selection were bred 

 in producing generation 27. Generations 29 to 40 inclusive 

 were raised in a room automatically maintained at 90° F. and 

 50 per cent relative humidity. In generations following the 

 32d, pressure of other work precluded the examination of all 

 the flies that hatched in a bottle, so that no more flies were ex- 

 amined in one generation than were required to find suitable 

 matings for the production of the next generation. Since the 

 first flies hatching in a bottle tend to have more bristles than 

 flies hatching later^ one could be reasonably sure of finding the 

 highest grade flies in the first few days of hatching. The result 

 of thus not counting all the flies that hatched, was to raise the 



Fig. 1 Frequency distributions of generations 12 to 49 in the high selected 

 race. The units on the base lines show the classes of extra bristles, the ordinates 

 represent frequencies. The series of curves at the right show the distributions 

 of all the flies in each generation; these frequencies are plotted on a percentage 

 basis so that the equal areas included by the curves in each generation do not 

 equal numbers of individuals. The curves at the left show the distributions of 

 a single line of families with only one pair of parents in each generation; the 

 ordinates for these curves are the actual numbers of flies. This single line is 

 the continuation of one called E in figure 6, page 81, MacDowell '15. The dis- 

 tributions of the parents chosen in the total generations are shown by small 

 curves between the larger ones; the parents chosen in the line of single families 

 are shown by squares (males), and triangles (females): solid lines — daughters 

 broken lines — sons. The pedigree numbers of the families in the single line, and 

 the numbers of families and flies in the different total generations are given on 

 the figure. 



» MacDowell: loc. cit., p. 86. 



