590 FRANK A. HAYS 
During the production of the majority of the Ist- and 5th- 
service litters the breeding animals were housed in somewhat 
cramped quarters. Conditions there were not conducive to the 
most rapid growth of the young and were not as favorable for the 
breeding females because of small space and rather poor ventila- 
tion and poor light. Furthermore, the progeny were crowded 
into rather limited exercising pens, and probably for this reason 
they did not develop at so rapid a rate as would have been the 
case under the more favorable quarters used later. The ma- 
jority of the 10th-service litters, on the other hand, and about 
half of the 15th-service litters were produced while the stock was 
housed in more ample quarters where the space was large, the 
ventilation good, and everything was conducive to health and 
thriftiness. In fact, the quarters used at that time were prac- 
tically as good as the present permanent and excellent quarters 
where the 20th-service litters were produced. The superior en- 
vironment of the advanced service litters is no doubt partly re- 
sponsible for the greater growth of the advanced service litters 
compared with the moderate service litters; but environment 
cannot be entirely the cause of the superiority of the 20th-service 
litters over the 15th, and the 15th-service litters over the 10th- 
service litters. Let us therefore seek a further explanation. 
Parentage may be an important factor affecting the weight. 
As has been previously noted, the variability of the female breed- 
ing stock is considerable, the range of weight was from 2500 to 
3250 grams, averaging 3050 grams, but the females have been 
so distributed among the three breeding males as to make three 
groups of practically uniform weight and variability in size. 
Nevertheless, lack of uniform weights in the progeny may still 
be partly due to variability of the female breeding stock. 
The size of the sire may also be a factor in controlling individual 
mean weight. The three sires used were quite different in weight; 
their weights are as follows: No. 1, 2850 grams; No. 3, 2575 grams, 
and No. 4, 2225 grams in ordinary breeding condition. Male 
No. 1 sired eleven of the seventeen litters included in the 20th- 
service group. He, being the largest of the three males, would be 
expected to sire the heaviest offspring at birth, and such offspring 
