328 
GENETICS: S. WRIGHT 
Proc. N. a. S. 
sidering the relations between parent and offspring as shown diagrammat- 
ically in figure 5. 
In a broad sense, the peculiarities of an individual are entirely determined 
by heredity and environment. In the present and doubtless most other 
cases, the latter should be divided into two elements. Nearly all tangible 
environmental conditions — feed, weather, health of dam, etc., are identical 
for litter mates. There may, indeed, be some differences in the blood sup- 
ply, but in the main, differences which are not genetic must be due to ir- 
regularities in development due to the intangible sort of causes to which 
FIG. 5. 
Diagram illustrating the casual relations between litter mates (O, O') and between 
each of them and their parents. H, H', H", H/" represent the genetic constitutions of 
the four individuals, G, G', G", and G'" that of four germ cells. E represents such 
environmental factors as are common to litter mates. D represents other factors, 
largely ontogenetic irregularity. The small letters stand for the various path 
coefficients. 
the word chance is applied. Variations which are due to inaccuracy in 
grading and are, therefore, merely apparent, cannot, however, be dis- 
tinguished from those of this class. Checks have been made which indi- 
cate that apparent variability due to this cause is small in comparison with 
the large amount of variability which is found to be due neither to heredity 
nor to tangible environmental conditions. 
