numerical ratio in the following generation follows according to the law of probability. Thus is explained both the fortuitous element that differentiates these cases from exact chemical combinations, and the definite numerical relations that appear in the aggregate of individuals. Grandparents AG (white) AB (white) AY (white) CB (black) | | | | +---------+ +-----------+ | | Parents AGAB (white) AYCB (yellow) | | +----------------------+ | Observed Calculated {AGAY} {ABAY} (White) 81 76 {AGAB} Offspring ---------------{ABAB} { {AGCY} (Yellow) 34 38 {ABCY} { {ABCB (Black) 20 19 {AGCB (Gray) 16 19 ---- ---- 151 152 Now, the point that I desire to emphasize is that one or two very simple mechanistic assumptions give a luminously clear explanation of the behavior of the hereditary characters according to Mendel's law, and at one stroke bring order out of the chaos in which facts of this kind at first sight seem to be. Not less significant is the fact that direct microscopical investigation is actually revealing in the germ-cells a physical mechanism that seems adequate to explain the disjunction of characters on which Mendel's law depends; and this mechanism probably gives us also at least a key to the long standing riddle of the determination and heredity of sex. These phenomena are therefore becoming intelligible from the mechanistic point of view. From any other they appear as an insoluble enigma. When such progress as this is being made, have we not a right to believe that we are employing a useful working hypothesis?