264 Evolution and Adaptation 



deviation is of great importance, because it is the index of 

 variability." x 



Of the different kinds of polygons there are two -main 

 sorts, the simple and the complex. The former have only a 

 single mode, the latter have more than one mode. Some 

 simple polygons lie symmetrically on each side of the mode, 

 Figure 3 A; others are unsymmetrical or skew, Figure 3 B. 

 The skew polygon generally extends out on one side farther 

 than on the other. It has been suggested that when a poly- 

 gon is symmetrical the species is not changing, and when 

 skew that the species is evolving in the direction of the 

 longer base. This assumes that the sort of variation meas- 

 ured by these curves is of the kind of which evolution is 

 made up, but this is a question that we must further con- 

 sider. How far the change indicated by the skew curve 

 may be carried is also another point for further examination. 



A complex polygon of variation, Figure 3 D, has been some- 

 times interpreted to mean that two subgroups exist in a 

 species, as is well shown in the case of the rhinoceros beetle 

 described by Bateson. Two kinds of male individuals exist, 

 some with long horns, others with short horns ; each with a 

 mode of its own, the two polygons overlapping. Other com- 

 plex polygons may be due to changes occurring at different 

 times in the life of the individual, as old age, for example. 



If, instead of examining the variations of the individuals of 

 the race, we study the variations in the different organs of the 

 same individual, we find in many cases that certain organs 

 vary together. Thus the right and the left leg nearly always 

 vary in the same direction, also the first joints of the index 

 and middle fingers, and the stature and the forearm. On the 

 other hand, the length of the clavicle and that of the humerus 

 do not vary together to the same extent; and the breadth and 

 height of the skull even less so. 



1 Davenport, C. B., " The Statistical Study of Biological Problems," Popular 

 Science Monthly, September, 1900. 



