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His treatment of the data is also cautious but thorough, so that when he works his way to a conclusion it stands firmly established. The conclusions reached are numerous and important, but the one of greatest theoretical interest is this, that feeble-mindedness is inherited as a simple recessive Mendelian unit-character.

Only the test is of little use in practice, as fertility may be diminished in unbalanced unions in all possible degrees, according to the amount of difference between the parents. If this amount is slight, if for instance, only one unit-character causes the difference, the injury to fertility may, be so small as to be practically nothing.

In such ordinary cases it is obvious that each character of the pollen-parent is combined with the same character of the pistil-parent. There may be slight individual differences, but each unit character will become opposed to, and united with, the same unit-character in the other parent. In the offspring the units will thus be paired, each pair consisting of two equivalent units.

Unfortunately, such simple cases do not readily occur. Most species, and even many elementary species are distinguished by more than one quality. Varieties deviating only in one unit-character from the species, are more common.