Variation and Heredity 



265 



We may also study those cases in which a particular 

 organ is repeated a number of times in the same individual, 

 as are the leaves of a tree. If the leaves of the same tree are 

 examined in respect, for example, to the number of veins that 

 each contains, we find that the number varies, and that the 

 results give a variation polygon exactly like that when differ- 

 ent individuals are compared with one another. Let us take 

 the illustration given by Pearson. He counted the veins on 

 each side of the midrib of the leaves of the beech. If a 

 number of leaves be collected from one tree, and the same 

 number from another, and if all those having fifteen veins are 

 put in one vertical column, and all those with sixteen in an- 

 other, as shown in the following table, it will be found that 



each tree has a mode of its own. Thus in the first tree the 

 mode is represented by nine individuals having eighteen 

 veins, and in the second by nine individuals having fifteen 

 veins. So far as this character is concerned we might have 

 interchanged certain of the individual leaves, but we could 

 not have interchanged the two series. They are individual 

 to the two trees. Now in what does this individuality con- 

 sist ? Clearly there are most leaves in one tree with eighteen 

 ribs, and most in the other with fifteen ribs. 



If we contrast these results with those obtained by picking 

 at random a large number of leaves from different beech trees, 

 we have no longer types of individuals, but racial characters. 

 Pearson has given the following table to illustrate these points : 



Frequency of Different Types of Beech Leaves 



