EVOLUTION 317 



s 



Two general kinds of variations are recognized: (1) continuous 

 or fluctuating or Darwinian variation, and (2) discontinuous or 

 sport variation or mutation. Fluctuating variation may be 

 illustrated by stature. If the individual heights of 1,000 college 

 men are grouped into classes, and the results plotted on right- 

 angled coordinates, a graph such as shown in Fig. 181 is obtained. 

 A class contains all the individuals within certain limits of height. 

 Thus the first class (from the left in the figure) contains all whose 

 height is from 155 to 157 cm., the next class, those who measure 

 from 158 to 160 cm., etc. The size or frequency of each class is 

 measured in a vertical direction on the ordinate, and the value 

 of each class is recorded on the horizontal base line or abscissa. 

 The height class containing the largest number of individuals is 

 the mode, in this case, class 174. The average height, 175.33 cm., 

 is somewhat greater than the modal height, because, as compared 

 to the modal height, there are more taller individuals than 

 shorter. The curve produced by connecting the height polygons 

 is a smooth curve showing gradations between classes. This is a 

 general type of distribution found in fluctuating variations. 



A discontinuous variation in stature would be one falling out- 

 side the normal limits of variation and not connected with the 

 normal variation range by intermediate conditions. Thus, if the 

 group measured had included individuals of, say, 150 or 210 cm. 

 in height, their position in the group would be outside the limits 

 of the range of the rest of the group, to which they would not be 

 related or connected by intermediate groups. 



Darwin attached more importance to fluctuating variation as 

 the basis of evolution because he believed discontinuous varia- 

 tions occurred too rarely in nature to be of any considerable value 

 in bringing about the origin of new species. To illustrate his 

 interpretation of the action of natural selection, if we assume that 

 the plus variations of any given character, i.e., variations greater 

 than the mode, tend to improve the equipment of an animal in 

 the struggle for existence, there is a premium on "plus" char- 

 acters which makes them more valuable than "minus" charac- 

 ters. If plus individuals reproduce more of their own kind and 

 less of undesirable kinds, i.e., if the variations are inherited, the 

 minus variants would as a result of the struggle for existence 

 tend to be reproduced in smaller and smaller numbers, so that the 

 mode of the variation graph would in the course of time gradually 



