426 THE POPULAR SCIENCE MONTHLY 



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 combina- 

 tion 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. W r eismann postu- 

 lated a determinant in the germ for every character which is inde- 

 pendently 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 ab- 

 sent 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 re- 

 spect except the shapes of their pollen grains, one of them having long 



