GENETICS: W, E. CASTLE 
433 
that we assume it to be for each cow the same in the second lactation 
period as in the first. 
Table 2 shows that 20 pure-bred Holsteins of average age 4.0 years, 
when the second calf was born, produced an average of 9475 pounds of 
milk in the second lactation period. Table 3 shows that 8 pure-bred 
Guernseys of average age 3.8 years when the second calf was born, pro- 
duced an average of 5593 pounds of milk in the second lactation period. 
Table 1 shows that the 13 Fi cows being of average age 3.9 years when 
the second calf was born, produced an average of 8663 pounds of milk 
in their second lactation period. This is 812 pounds less than the aver- 
age production of the pure Holsteins, but is 3070 pounds more than the 
average production of the pure Guernseys and exceeds the intermediate 
between the pure breeds by 1129 pounds. The deviation is of the same 
sort as in the first lactation period but is even more striking. As regards 
quantity production it is clear that the Fi cows are better than an inter- 
mediate between the breeds crossed. 
In butter-fat production, using the same percentage estimates as were 
used for the first lactation period, the Holsteins average 322 pounds, 
the Guernseys 280 pounds, and the Fi cows 363 pounds, an excess for the 
Fi cows of 41 pounds over the pure Holsteins and of 83 pounds over the 
pure Guernseys. Again the Fi cows show superiority over either pure 
breed in butter-fat production, while in quantity production of milk they 
are nearer to pure Holsteins than to an intermediate between the pure 
breeds. 
This result is comparable with that observed in the inheritance of size 
and other quantitative characters both in animals and in plants. In 
such cases Fi usually surpasses more or less the intermediate between 
the races crossed owing to the superior vigor commonly possessed by 
cross-bred organisms. But F2 usually varies about a strict intermediate 
between the races crossed and such we may anticipate will probably be 
the outcome in this case unless quantity production and quality produc- 
tion to some extent depend on different genetic factors, the point which 
Mr. Bowlker set out to investigate. If independent genetic factors are 
really concerned in quantity production and in quality production re- 
spectively, their recombination in F2 may be expected in animals which 
are both high producers and which give rich milk and such animals should 
be capable of transmitting this desirable combination of characters to 
their descendants. Such a result would realize Mr. Bowlker's fondest 
hopes, and would open great possibilities for the systematic improvement 
of dairy cattle. 
