GROWTH AND MORPHOLOGICAL CHARACTERS. 55 



was obtained in the F2 generation. This they held due to a rearrange- 

 ment of a number of separable factors for the character involved. 



Tammes (1911) has likewise thought it possible to ascertain a number 

 of separate, independent factors for characters in species and varieties 

 of flax (Linmn) . She has calculated the approximate number of factors 

 for each character, such as length and breadth of the seeds, length and 

 breadth of the petals, color of the flowers, and dehiscence of the capsules. 

 The proportionate number of individuals in the F2 generation, which 

 show the pure parental character, was taken as an index of the number 

 of factors for that character. 



Phillips (1912) has recently crossed two races of ducks, differing 

 in size, and obtained an increase in variability in the F2 generation. 

 MacDowell (unpublished) had similar experience with rabbits. An 

 increase in variability in the F2 generation can not in itself be considered 

 a final criterion of Mendelizing inheritance, for the F2 individuals 

 should be tested in order to show that all do nat regress to the mean, 

 but some pure recombinations have been formed. Very little has been 

 done with F3 generations in such crosses. 



East and Emerson (1913) have continued their researches in maize 

 on the inheritance of number of rows per ear, length of ears, diameter 

 of ears, weight of seeds, breadth of seeds, height of plants, number of 

 nodes per stalk, internodal length, number of stalks per plant, total 

 length of stalks per plant, and duration of growth, and have given 

 evidence that the F2 generation is in general more variable than the 

 Fi or either parent. Furthermore, the F3 generations indicated that 

 the parental types recovered in the F2 might breed true, that inter- 

 mediate types with new modes had been obtained, and that some F2 

 individuals gave F3 progeny just as variable as the F2. They conclude 

 "that the results secured in the experiments with maize were what 

 might well be expected if quantitative differences were due to numer- 

 ous factors inherited in a strictly Mendelian manner." 



The striking similarity between these crosses and some of the well- 

 known color crosses makes it seem probable that both forms of inherit- 

 ance may be Mendelian; for in both the segregation is delayed until the 

 F2 generation. Nevertheless, the clearness shown in color-inheritance 

 does not stand out in size-inheritance. Interaction of many factors 

 and environmental effects may play a greater part. Whether or not 

 the general size of mammals will lend itself to such a solution is difficult 

 to say. There is much correlation in the size of parts, although we 

 do find that partially uncorrelated individual parts, such as short legs, 

 tails, or ears, may exist in mammals. It would be theoretically and 

 practically desirable to know whether the inheritance of the general 

 body size is Mendelian when mammals of the same proportions but 

 of (Afferent size are crossed. 



