STUDIES OF ADULT WEIGHT. 45 



their own fraternities, if we assume that gametes bearing characters 

 for different sizes do not indissolubly join their size-determining sub- 

 stances. If the variability of FI is really partly caused by a segrega- 

 tion, the difference between the variabilities of the two generations is 

 due to the fact that the parents of the heterozygous mothers differed 

 in fewer factors than did the parents of the FI generation. If the mixed 

 nature of the females' pedigrees is claimed to influence the results, it 

 can only make the difference in variability more significant. 



This work and that of Philips (1912) on ducks show that in animals 

 quantitative characters may give increased variability in the second 

 generation, as has been previously shown to be true for plants. Many 

 of the cases that have been presented have similarities. They all deal 

 with heritable characters, quantitatively different. To a greater or less 

 degree, they are subject to fluctuations that, not being heritable, may 

 be roughly ascribed to environment. When crosses between extremes 

 are made, hybrids are formed generally of an intermediate nature. In 

 the following generation new forms appear that are similar to the 

 original parents or even more extreme, although the majority of the 

 individuals are intermediate. In certain cases crosses between similar 

 varieties or lines, after an FI like the parents, give second generations 

 in which a wide range of grades is found. 



These are the facts and this is all that can be definitely ascertained 

 from the work that has already been completed. Having been found 

 to occur a few times, it is supposed that these phenomena will always 

 be the same. Yet we know that it is only a probability. On this 

 probability we build a law, a general statement or description of the 

 facts, which stands until new facts demand a new statement. In this 

 case such a law may be stated: 



The second generation of a size cross shoivs greater diversity than does 

 the first generation or the parental lines. Size characters that are inherited 

 in a Mendelian fashion are included in the above statement. 



All practical application must come from this fact. 



THEORETICAL. 



When the reason for this increased variability is asked or when its 

 mechanism is suggested we reach the realm of hypothesis and supposi- 

 tion, yet very little interest is found in facts unrelated to explanatory 

 theories. Mendel's papers passed through a long dormant period; his 

 letters to Nageli failed to excite interest. When their theoretical sig- 

 nificance was once realized, they received wide attention. 



The theory of multiple units has been developed in Part I. From 

 cases of color inheritance in which very definite ratios suggest the 

 presence of two or more independent units we came to cases of struc- 

 tural characters which give similar ratios to their absence (ligulaB, Ris- 

 pentypen). So we have the same basis for assuming two or more 



