152 FAMILY STUDIES IN BUILD. 



inheritance. More than that, however, the theory of multiple factors 

 explains a number of phenomena that the theory of blending inher- 

 itance does not, such as the greater variability of the F 2 as contrasted 

 with the Fi generation, and the skewness in distribution of progeny 

 in the back-cross. Thus an F X S mating should produce a "medium" 

 progeny with symmetrical distribution of deviating types on the 

 theory of blending inheritance; actually it produces not only me- 

 diums, but an excess of fleshy over slender, which is readily accounted 

 for on the hypothesis that fleshiness is partially dominant over slen- 

 derness. In this and other ways, especially in focussing attention on 

 gametes, the doctrine of multiple factors is essential to an adequate 

 interpretation of the results of mating between persons of dissimilar 

 build. 



D. SUMMARY OF CONCLUSIONS. 



1. Two types of variation in build are to be distinguished: (1) the 

 change in average build that accompanies ontogeny and (2) variations 

 in adult build. 



2. The popular idea of build is best expressed as the ratio of trans- 

 verse chest diameter to stature, or (since the chest diameter is rarely 

 known) chest-girth to stature. When only weight and stature are 

 known, the closest approximation to the chests-stature ratio is given 

 by the weights- (stature) 2 ratio, and this is taken as the standard 

 index of build. 



3. The correlation between the standard index of build and the 

 relative chest-girth is, for males, about 0.45. 



4. The index of build of adult males is slightly greater than of 

 females, because of the relatively greater chest-girth of males. The 

 average index of build for males is 2.52 (35.8) and for females 

 2.43 (34.5). 



5. There are marked racial differences in build; but they are not 

 so great as the differences in ontogenetic stages. 



6. There are geographical differences in build; the heavy build of 

 northern peoples may be due to a physiological reaction or, in part, 

 to a selective survival of the fleshier individuals or strains. 



7. The ontogenetic curve of build, expressed by relative chest-girth, 

 is expressed by figure 1 and (for infancy) figure 2. It shows that at 

 birth chest-girth is about two-thirds of stature and diminishes in the 

 male to the age of 12 years; thereafter, on the average, it rises to 

 complete maturity. 



8. Build declines temporarily during the first month of life, owing 

 to the physiological difficulties attending adjustment to new condi- 

 tions. It declines temporarily, again, at about 8 months, probably 

 due to the cutting of the incisor teeth. 



