440 



ELEMENTARY BIOLOGY 



Careful observers familiar with farm life and practice have 

 noted that cultivated races of plants and animals tend to 

 deteriorate markedly in a very few generations unless the 

 selections are made every year. The explanation for this was 

 unknown, but the fact was clear, and its application was found 

 in steadily selecting the best or most desirable individuals or 



FIG. 235. Improved varieties of domestic birds 



seeds for further propagation. For one thing was certain : it 

 was possible to improve the stocks to a considerable degree 

 by constant selection (see Figs. 235, 236). 



466. Mixed types. In the diagrams used to illustrate the 

 measurement of variations (Figs. 231-234) we see that for 

 every series there is one measurement that represents the 

 condition of a comparatively large number of individuals. 

 This appears in the diagram as the peak of the curve. In 

 many groups of individuals careful measurements will show 

 two such peaks. For example, the Dutch botanist Hugo de 

 Vries counted the number of ray-florets in a species of daisy 

 (Chrysanthemum segetuni) and plotted the results of his count- 

 ing, which are given graphically in Fig. 237. 



