PHYSICAL CHARACTERS IN MAN 33 



pendently of the rest of the skeleton . Two such groups 

 are skull length and breadth and the three leg bones. 

 Again, femur and tibia form a group subject to 

 common influences which do not affect the humerus 

 (foreleg). How far these variations were controlled 

 by genetic factors is of course unknown. 



Castle specifically confines to animals his view that 

 all size factors are general, excluding it from plants 

 on the ground of a greater hormone control in animals. 

 But the apparent difference may simply be due to 

 the present state of our knowledge. In plants it 

 has been shown (Gates, 191 7) that when a large- 

 flowered species is crossed with a small-flowered one, 

 an intermediate F^ ma}" be followed by later genera- 

 tions in which widely different sizes of flower occur 

 simultaneously on the same plant, and even different 

 lengths of petal in the same flower. This striking 

 result, which has been studied on a large scale in 

 CEnothera crosses, shows that in plants, at any rate, 

 organs of widely different size ma}" occur on the same 

 individual as the result of inherited differences. 



Another important fact which bears on the whole 

 theory of multiple factors in the interpretation of 

 size inheritance has been brought out by Sumner 

 and Huestis (1921). From extensive measurements 

 of the right and left mandibles, femurs, and pelvic 

 bones of the Californian deer mouse, Peromysciis 

 maniculatiis , they have constructed curves for the 

 sinistro-dextral ratio for each bone — i.e., the relative 

 lengths of the corresponding right and left bones. 

 The range of variation on either side of equality in 

 this ratio is, of course, small in every case, but they 

 are able to show statistically that the difference — 

 i.e., excess of length or weight on the right or left 

 side — is, as might be expected, not inherited from one 

 generation to the next. Nevertheless, if pure races 

 are compared with hybrids, the Y^ generation shows 



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