34 EVOLUTION OP LIVING ORGANISMS. 



every sort of character, structural or functional. Shape, 

 size, colour, scent, number and relative position of parts, 

 are all subject to variation. The most complex capaci- 

 ties, such as fertility, the power of resistance to disease, 

 and intellectual ability are notoriously variable. More- 

 over, variation occurs at every stage of life : the seed, 

 seedling, young and adult plant, the egg, embryo, young 

 and adult animal, all show variations. 



These variations can often be accurately measured, and 

 the statistical study of variation begun by Qnetelet and 

 Galton, and carried on by W. F. R Weldon, K. Pearson, 

 and others, has yielded many important results. If we 

 took a number of sticks picked up at random, and 

 measured their length, we should find that while sticks 

 of medium length are the most numerous, the sticks 

 become rarer and rarer as they approach the extreme of 

 length and shortness. If the number of sticks be large 

 enough, the length occurring most frequently (the modal 

 value or mode) would be found at or near the mean 

 between the two extremes. In fact the length is, as we 

 say, determined by chance, and the results obey the laws 

 of probability, being due to a number of independent 

 causes acting at random on the individual sticks. It is 

 the same with variations, as shown by the examples on 

 the following page. 



These results can be graphically represented in the form 

 of a curve, which will be found to agree with a normal 

 curve of error. The vertical passing through the apex of 

 the curve will represent the mode, and the distance from 

 it along the base will give the range of variation.* 



Asymmetrical or skew curves occur when for some 

 reason the variation is more limited on one side than the 

 other. 



These, then, are the materials from which natural 

 selection has to choose ; but to understand their nature 



* The reader is referred to Mr. Watson's volume on Heredity, in this 

 series, for a fuller account of variation and its mathematical treatment. 



