302 



FOUNDATIONS OF BIOLOGY 



ously? The experience of practical breeders gives a partial 

 answer, since the continual selection of the best animals for 

 mating and the best plants for seed has been a profitable 

 procedure. But it has long been known that after a certain 

 amount of selection has been practiced it ceases to be so 





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Inoktf 64 55 56 57 58 59 61 62 63 64 65 66 67 68 69 10 7J 

 Pergont 3 3 7 18 34 80 135 163 183^163 115 78 41 ' IB 6 5 & 



FIG. 155. Normal frequency curve. Plotted measurements of the height of 

 1,052 women. The height of each rectangle is proportional to the number of individuals 

 of each given height. (Cf. Figs. 153, 154.) (From Kellicott, after Pearson.) 



effective, and thenceforth serves chiefly to keep the character 

 at the higher level attained. (Fig. 156.) 



The crux of the matter is in regard to exactly what the 

 fluctuations are. Modifications (non-heritable) and fre- 

 quently combinations (heritable) give a normal variability 

 curve, and both may be included in fluctuations. This mix- 

 ture of heritable and non-heritable variations is what makes 

 confusion. If we rule out combinations, by inbreeding 

 or by self-fertilization of homozygous individuals establish 

 PURE LINES then the fluctuations are all modifications and 

 selection is ineffectual with characters which are not inherited. 



