CHROMOSOMES AND MEN DELI AN HEREDITY 309 



treated as the differential element in a large system of factors. "The 

 factorial hypothesis does not assume that any one factor produces a 

 character directly and by itself, but only that a character in one organism 

 may differ from a character in another because the sets of factors in the 

 two organisms have one difference" (Morgan et al., 1915). 



Bridges (1923) has described the supposed activity of genes and the 

 principle of "genie balance" as follows: 



. . . the genes of the entire complement act together, since all are liberating 

 their chemical products into the common cytoplasm. But since the products 

 of the different genes are different, they take effect in different ways upon the 

 developing organism. Thus some of the genes will have much effect upon one 

 character but little effect upon another. Each character will be determined 

 by all the genes, but in each case most of the effect will be produced by a much 

 more limited number. Each particular character shows a grade of development 

 corresponding to the number and effectiveness of the genes that are producing 

 it. In the case of many of the characters there are genes tending to make the 

 character less pronounced, and the grade realized is that corresponding to the 

 equilibrium between these plus and minus modifiers. The directions of modi- 

 fication for each character are in general quite diverse, and it is somewhat for 

 the sake of simplicity that they are lumped together as plus and minus modifiers. 



It is thus thought that the genes affect the characters mainly by 

 influencing the course of metabolism and the processes of differentiation 

 during ontogeny. Very little is known on this subject. It is thought 

 that a few hundred genes can produce a much larger number of distinct 

 effects upon characters because they act in different combinations and 

 probably in different ways at different stages of development. Their 

 actions are in part responses to changing conditions in the cytoplasm 

 and external environment, i.e., in the other portions of the system of 

 which they are a part. Still less is known about the functions of genes 

 in the mature organism, although a beginning has been made on this 

 problem. 



How the influences of genes are transmitted to the cell or tissue in 

 which they lie is at present wholly a matter of conjecture. It has been 

 held that certain products of genie activity are liberated into the cyto- 

 plasm at the time of mitosis; this is suggested by the time of appearance 

 of differentiated regions in certain animal eggs (p. 419). It has also been 

 thought that nucleolar matter so liberated may possibly carry such 

 products (p. ]20). In this connection it should be remembered that 

 many tissues undergo a considerable amount of growth and differentiation 

 and perform their characteristic functions long after all the nuclear and 

 cell-divisions are completed, which strongly suggests that genes exert 

 influences upon development and behavior while the nucleus is in the 

 active metabolic stage. 



