A complex polygon of variation, Figure 3 D, has been sometimes 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 complex 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.
════════════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════╤════ No. of Veins│ 10│ 11│ 12│ 13│ 14│ 15│ 16│ 17│ 18│ 19│ 20│ 21│ 22 ────────────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼────┼──── First Tree │ —│ —│ —│ —│ —│ 1│ 4│ 7│ 9│ 4│ 1│ —│ — Second Tree │ —│ —│ —│ 3│ 4│ 9│ 8│ 2│ —│ —│ —│ —│ — ════════════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════╧════
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 different 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 another, 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 consist? Clearly there are most leaves in one tree with eighteen ribs, and most in the other with fifteen ribs.
