as the product of the development of a single determiner; undoubtedly many causes are involved in the development of every character, but the differential cause or combination of causes is that which is peculiar to the development of each particular character.
Again it is not necessary to suppose that every developed character is represented in the germ by a distinct determiner, or inheritance unit, just as it is not necessary to suppose that every chemical compound contains a peculiar chemical element; but it is necessary to suppose that each hereditary character is caused by some particular combination of inheritance units and that each compound is produced by some particular combination of chemical elements. An enormous number of chemical compounds exists as the result of various combinations of some eighty different elements, and an almost endless number of words and combinations of words—indeed, whole literatures—may be made with the twenty-six letters of the alphabet. It is quite probable that the kinds of inheritance units are few in number as compared with the multitudes of adult characters, and that different combinations of the units give rise to different adult characters; but it is certain that every inherited difference in adult organization must have had some differential cause or factor in germinal organization.
Mendel did not speculate about the nature of hereditary units, though he evidently conceived that there was something in the germ which corresponded to each character of the plant. Weismann postulated a determinant in the germ for every character which is independently heritable, and many recent students of heredity hold a similar view.
But it is evident that there is not an exact one-to-one correspondence of inheritance units and adult characters. Many characters may be decided by a single unit or factor; for example, all the numerous secondary sexual characters which distinguish males from females are decided by the original factor which determines whether the germ cells shall be ova or spermatozoa.
On the other hand, two or more factors may be concerned in the production of a' single character. In many cases among both plants and animals the development of color appears to depend upon the presence in the germ cells and the cooperation in development of at least two factors, viz. (1) a pigment factor P (for black B, for brown Br, for yellow Y, for red R, etc.), and (2) a color developer C. When both of these factors are present color develops; when either one is absent no color appears.
Such cases have been described for mice, guinea-pigs and rabbits as well as for several species of plants. Bateson and Punnett found two varieties of white sweet peas which were apparently alike in every respect except the shapes of their pollen grains, one of them having long
