PRINCIPLES OF LIVESTOCK BREEDING. 9 
ages in heavy horses are 15 to 20 years, in cattle 10 to 15 years, in 
sheep 7 to 10 years, in hogs 5 to 8 years. 
Examples of breeding at much greater ages can, of course, be found. 
A ewe is recorded as lambing at 19 years of age. As for the maximum 
length of life, horses are reported to have passed 40 years, cattle 30 
years, sheep 20 years. 
Chickens hatched early in spring usually begin to lay in from 6 to 
8 months, but cases of laying at less than 5 months are on record. 
The best egg record is usually produced in the first full year of laying. 
The second and often the third years are nearly as good in the egg 
breeds. After this the number declines, but laying may continue to 
the eighth or ninth year. 
That heredity is a factor in determining fertility may be seen by 
comparing the different breeds of hogs and sheep. Among hogs, the 
bacon breeds, as the Tamworth and the large Yorkshires, have con- 
siderably larger litters than the lard breeds. There are differences 
among the latter. In very extensive tabulations made by G. M. 
Rommel the average for Poland Chinas was 7.52, compared with 9.26 
for Duroc-Jerseys. Averages for the other important lard breeds are 
intermediate. Among sheep, certain breeds, such as the Dorsets, 
Oxford Downs, and Shropshires, have twins more frequently than 
singles under favorable conditions. Southdowns are distinctly less 
fecund, while among Merinos twins are not common. Within a 
breed, the number at a birth is determined by such a variety of factors 
that it is not easy to demonstrate the influence of heredity. Never- 
theless it has been shown for Poland China hogs (Rommel and 
Phillips) and for Shropshire sheep (Rietz and Roberts) that females 
born in large litters in the former and as twins in the latter have a 
slight tendency to produce more at a birth than the average. 
Differences in fertility appear to be so great among individuals that 
breeders often look for some outer indication. It is generally believed 
that strong development of the masculine and feminine types in 
males and females, respectively, gives a special indication of fertility 
as well as of general vigor. It is sometimes thought that the fertil- 
ity of a female is indicated by the number of mammae. Professor 
Pearl has shown that this is true in a general way in a comparison of 
different kinds of mammals. It does not, however, seem to hold to 
any significant extent within a single species. Thus Alexander 
Graham Bell was able to increase the number of functional nipples 
in his flock of sheep from 2 to 6 by careful' selection. This change 
was not accompanied to any appreciable extent by increase in the 
percentage of twins, although it was an advantage in those cases in 
which twins were born. Similarly, Pearl has shown that there is no 
significant correlation between the number of mammae of a sow and 
the size of her litter. 
5254°— 20— Bull. 905 2 
