128 Shull. 



the segregation of Mendelian characters produces some pure homo- 

 zygotes and some individuals which have as many heterozygous 

 elements as there were in the Fi individuals, i. e. there are some un- 

 stimulated individuals and some which are highly stimulated, as well 

 as individuals having all intermediate degrees of stimulation. The 

 phenomenon of heterosis alone, therefore, in so far as it arises from 

 Mendelian differences, will cause an increased variability in size-relations 

 in the F2 as compared with the Pi and the Fi generations. 



Furthermore, those F2 individuals which owe their differences of 

 size and form to their different degrees of heterosis, will yield Fs families 

 also showing different average heights or different average sizes of any 

 organ which may be under consideration, because such families will possess 

 different average degrees of stimulation. Such F 3 families thus exhibit 

 apparent differentiation in size-characters wholly aside from the existence 

 and segregation of specific size-determiners. That some of these differ- 

 entiating genes whose heterozygous condition stimulates to greater 

 physiological vigor are specific modifiers of size, form or function of 

 the organism, or of one or more of its parts, is extremely probable, 

 / ^ut this fact can be demonstrated beyond question only by comparison 

 among extracted homozygous types. 



In most of the work on increased F 2 variability, the facts here 

 stressed seem to have been left entirely out of account, namely, that 

 every criterion given for the segregation of continuously variable size- 

 characters, is also produced, to a certain extent at least, by heterosis 

 from Mendelian determiners which, in their homozygous condition, 

 are not necessarily productive of size-differences. Only in one instance 

 have I found any mention of these particular effects of heterosis, HAYES, 

 EAST and BEINHAKT (1913, p. 55) having recently assigned them as a 

 reason why "the coefficient of variability is not a very safe criterion 

 by which to judge when dealing with a character such as area of leaves." 



That the occurrence of heterosis increases the difficulty of genetic 

 analysis of size-characters in another way, namely, by throwing the 

 stimulated individuals into size-classes in which they would not belong 

 if not thus stimulated, has been recognized by several investigators, 

 particularly by EAST and HAYES (1911), HAYES (1912) and EMERSON 

 and EAST (1913) and these authors have for this reason laid stress on 

 certain cases in which the Fi hybrids do not exceed the average of 



branches may differ from those on the branches which usually develop. The increased 

 variability in flower-size in GOODSPEED'S (1912, 1913) F 1 Nicotiana-liybrids may perhaps 

 be accounted for in this way. 



