This difficulty arises from the fact that we are still ignorant of the exact laws of inheritance. Just how mental characteristics are transmitted from parent to child is not yet definitely known. It therefore becomes a serious matter to decide beforehand that such and such a person who has mental defect would certainly transmit the same defect to his offspring and that consequently he ought not to be allowed to have offspring. THE MENDELIAN LAW In 1866 an Austrian monk by the name of Gregor Mendel discovered and published a law of inheritance in certain plants, which, after lying practically unknown for nearly forty years, was rediscovered in 1900 and since then has been tested with regard to a great many plants and animals. Mendel found that there were certain peculiarities in plants which he termed “unit characters” that were transmitted from parent to offspring in a definite way. His classical work was on the propagation of the ordinary garden pea, in which case he found that a quality like tallness, as contrasted with dwarfness, was transmitted as follows:-- If tall and dwarf peas were crossed, he found in the first generation nothing but tall peas. But if these peas were allowed to grow and fertilize themselves, in the next generation he got tall and dwarf peas in the ratio of three to one. The dwarf peas in this case bred true, _i.e._ when they were planted by themselves and self-fertilized there was never anything but dwarf peas, no matter how many generations were tested. On the other hand, the tall peas were divisible by experiment into two groups; first, those that always bred true, viz. always tall peas; and secondly, another group that bred tall and dwarf in the same ratio of three to one; and from these the same cycle was repeated. Mendel called the character, which did _not_ appear in the first generation (dwarfness), “recessive”; the other (tallness) he called “dominant.” The recessive factor is now generally considered to be due to the absence of something which, if present, would give the dominant factor. According to this view, dwarfness is simply the absence of tallness. This law has been found to hold true for many unit characters in many plants and animals. Since study in human heredity has been taken up, it has been a natural question, Does this same law apply to human beings? It has been found that it does apply in the case of many qualities, like color of hair, albinism, brachydactylism, and other peculiarities. Investigation has of late been extended to mental conditions. Rosanoff has shown pretty clearly that the law applies in the case of insanity, while Davenport and Weeks have shown evidence that it applies in cases of epilepsy.