Experience has shown that without any doubt there are conditions for which defective heredity is largely responsible. It is often difficult to determine the actual rôle which this plays in a given case. Efforts have been made to reduce the study of these factors to a definite scientific basis. In 1865 Gregor Mendel,[62] Abbot of Brünn, published an account of a series of experiments made by him with the common pea (pisum sativum) which was destined to revolutionize our views on the subject of heredity. On crossing a tall with a dwarf plant, tall hybrids resulted with no intermediate forms. This inheritance is said to be due to the presence of a definite "determiner" in the germ plasm. All of his hybrids being of the tall variety, he designated that character as the "dominant," the dwarf being spoken of as the "recessive." On the fertilization of these hybrids he obtained another generation, which averages three tall plants to one dwarf. Further investigation showed that the dwarfs always bred true, as did about one out of three of the tall varieties, the remaining two behaving as did the original hybrids and giving three talls to one dwarf. He therefore observed that he was dealing with three varieties of inheritance, the dwarfs which bred true, the talls which bred true and the talls with a fixed proportion of talls and dwarfs. The phenomenon as noted by Mendel is not, however, universal in its application. Curiously enough no attention was given to Mendel's experiments until eighteen years after his death, when his work was rediscovered by de Vries, Correns and Tschermak in 1900. Davenport[63] has shown that there are six possible matings of germ cells as illustrated by the pigment of the eye:—1. Both parents, pigmented iris (brown eyes) and duplex—all offspring with pigmented iris and duplex; 2. Both parents brown-eyed, one duplex, one simplex—all children brown-eyed, but half simplex; 3. One parent brown-eyed and duplex, the other blue-eyed—all children brown-eyed and simplex; 4. Both parents brown-eyed and simplex—one-fourth of the children brown-eyed and duplex, one-half brown-eyed and simplex, and one-fourth blue-eyed; 5. One parent brown-eyed and simplex, and the other blue-eyed—one-half the children brown-eyed and simplex, the other half blue-eyed; 6. Both parents blue-eyed—all children blue-eyed. It should be explained that a duplex origin means the inheritance of a character from both parents and simplex from only one. The principles of the Mendelian laws of heredity have been applied to a study of the color of the eyes and skin, the color and form of the hair, the stature, body weight and many other family traits such as musical knowledge, ability along artistic and literary lines, mechanical skill, etc. They have also been applied to the study of various diseases, such as Huntington's chorea, hereditary ataxia, deaf-mutism, feeblemindedness, epilepsy and insanity, etc.