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THE AMEBIC AN NATURALIST [Vol. XLVIII 



the character "hoodedness" belongs in this class and that the 

 ancestral hooded rats used by Castle and Phillips were the de- 

 scendants of a cross involving many genes for that character. 



The results of such a cross are of course complicated, for the 

 different pairs of allelomorphs generally can undergo recombina- 

 tion at the reduction division of the hybrid, so that in F 2 or 

 subsequent generations as many different genetic types of indi- 

 viduals are formed as there are possible different combinations 

 of those factors wherein the ancestors differed. Not all these 

 genetic types, of course, will fall into different phenotypes, yet 

 generally there will be a large number of overlapping pheno- 

 types among the progeny. 



The larger the number of factors in which the two ancestral 

 lines differed, the larger will be the number of different possible 

 combinations of these factors, and accordingly the smaller will 

 be the chance of any individual having one of those particular 

 combinations necessary to a relatively high or a relatively low 

 intensity of the character. In other words, the larger the num- 

 ber of factors (for one character) for which a population is 

 heterogeneous, the more numerous are the possible different 

 grades of intensity of this character among the different indi- 

 viduals, but the fewer will be the individuals which approach the 

 more extreme grades theoretically possible in such a population. 3 

 Suppose, for example, that two parents differ in five pairs of 

 factors for hoodedness, which are partially dominant 4 to their 

 allelomorphs and summative in their action. Then in F 2 not one 

 influenced by diverse means, merely because one of the two races had been 

 subjected to a very long and drastic selection, so that any of those rare 



tor may dominate, in others the "plus"), the rigor of selection will be di- 

 minM„.,l. si,,,-,. l„>r,.r<, Z yjj,,i.s forms ran nut <ii>t iny.ilshed from homozy- 



