576 
Journal of Agricultural Research 
Vol. XXIII, No. 7 
Tabee VIII.— Variability of the F x and F 2 generations —Continued. 
Fi 
F* 
Age. 
Num¬ 
ber of 
indi¬ 
viduals. 
Mean 
weight. 
Standard 
deviation. 
Coeffi¬ 
cient of 
varia¬ 
bility. 
Num¬ 
ber of 
indi¬ 
viduals. 
Mean 
weight. 
Standard 
deviation. 
Coeffi- 
cent of 
vari- 
bility. 
Six months. 
15 
Pounds. 
86.0 
19. Oi 
22. I 
48 
Pounds. 
87-54 
25. 76 
29 * 43 
Seven months. 
I 5 
115.6 
30.04 
2 5 - 99 
43 
113.0 
28. 58 
25. 29 
Eight months. 
13 
135 - 61 
33- *8 
24. 47 
43 
130.46 
36.0 
27 - 59 
Nine months. 
*3 
164. 31 
32.32 
19. 67 
32 
r 37 -19 
3 8 . 7 I 
28. 22 
Ten months. 
*3 
189.23 
29. 19 
I 5-43 
33 
r 59 - 27 
37-71 
23. 68 
Eleven months. 
12 
197 - 25 
28.38 
i 4.39 
19 
166. 79 
48.12 
28. 85 
Twelve months.... 
12 
218. 92 
38 . 95 
* 7 - 79 
11 
181. 55 
36- 5 
20.1 
Thirteen months... 
11 
225. 36 
29. 05 
12. 89 
9 
202. 33 
40.83 
20.1 
Fourteen months... 
10 
229.5 
34 -1 
14. 86 
7 
223 .57 
32-25 
14.43 
Fifteen months.... 
Sixteen months.... 
10 
10 
226. 7 
242. 8 
41.1 
41. 1 
18-13 
l6. 92 
2 
J 95 
The outstanding fact of genetic importance is that the variability of 
the F 2 generation is distinctly greater than that of F r That, to be sure, 
is what was to be expected from the genetic standpoint, and it is in agree¬ 
ment with the experience of practical breeders. It is definite proof of 
two facts: First, some of the factors which have the power to stimulate 
growth are not identical in the Duroc-Jersey and the Berkshire breeds; 
second, there is some degree of homozygosis for these growth-stimu¬ 
lating factors within the limits of each breed. To what extent being 
pure-bred from the standpoint of the breed registry society indicates 
homozygosis for these growth factors; whether the number of factors in¬ 
volved can be determined; whether the differences of growth indicate 
differences in the identity of the factors or merely differences in the com¬ 
binations; and whether these factors are so linked in groups as to make 
recombination of the desirable ones impossible, or at least impracticable— 
these are all interesting questions which these data raise but are too 
meager to answer. 
Other interesting questions which are not, however, primarily genetic, 
relate to the shape of the curve. The maximum variability seems to be 
reached at or shortly following weaning time and coincides with the 
period of the lowest mortality. Does this mean that the best time to 
select breeding stock for size and early maturity is when they are from 
3 to 6 months old? Of course it was impracticable to keep the hogs, 
which were not desired for breeding purposes, until they were 2 or 3 
years old, and therefore there is still left open the question as to whether 
the hogs would not have finally become more uniform in size; that is, 
as to whether the differences were in potential size or in earliness of 
maturity. 
Plate 2, K, is a photograph of one of the F 2 litters most extreme in 
its range of variation in growthiness. These four individuals had received 
identical treatment, and none of them had ever been recorded as sick, 
yet when this photograph was taken, when they were being marketed at 
the age of 11 months, the weights were, from left to right, 221, 293, 90, 
and 170 pounds. Possibly more animals are bora runts than is com¬ 
monly believed. 
