EVOLUTION, VARIATION AND HEREDITY 



459 



mals and plants under natural conditions. No two members of the 

 same species are absolutely alike ; they vary in size, shape, colour, 

 relative size of parts, and so on. Not only is this true of the external 

 features, but it applies also to the various internal organs. All 

 parts of. animals vary to some extent, and while in some cases it is 

 more apparent than in others, it is almost always possible to measure 

 this variability. This statistical investigation of the phenomenon 

 of variation was started in England by Francis Galton and continued 

 by Professors Karl Pearson and Weldon. 



Let us take an example or two. 



A collection of a large number of beech leaves was made at 

 random and the number of main veins coming off from the mid 

 rib counted, and the following result obtained : 



This may also be expressed in the form of a curve with the number 

 of veins on the abscissa and the number of leaves on the ordinate. 



A similar series of measurements have been made on men, i.e. the 

 height of 4426 members of the University of Cambridge of British 

 extraction. The result is shown in the accompanying diagram, 

 where the stature in inches is on the abscissa and the number of indi- 

 viduals on the ordinate. The diagram also shows the curve that 

 most nearly fits the points obtained. (Such a curve is known as a 

 normal curve, and is expressed mathematically by the equation 



FIG. 164. Curve of heights of undergraduates of Cambridge University 

 of British extraction. 



