264 Evolution and Adaptation 
deviation is of great importance, because it is the index of 
variability.” 
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, 
