PRINCIPLES OF LIVESTOCK BREEDING. 
45 
to produce lambs out of season. Under some circumstances dairy 
cows can well be bred to bulls of the beef breeds in order to produce 
calves which can be fattened profitably. In some cases it may even 
be worth while to develop races within two breeds specially designed 
to be complementary to each other in crosses. The danger in any 
system of crossbreeding is that the very excellence of the first genera- 
tion will tempt the breeder to use them as breeding stock. The 
additional vigor due to crossing decreases after the first generation 
and uniformity of type is lost at once. 
IMPROVEMENT. 
There is a certain antagonism between control over heredity and 
radical improvement. Perfect control over heredity implies the 
absence of all variation among the progeny of a mating. A useful 
new type is most likely to be found where there is a maximum of 
variation. Thus the pioneer breeder must make wide crosses. The 
first generation may be expected to be about intermediate between 
Fig. 11.— A group of Corriedale rams. This breed originated in Australia in the crossing of Lincoln or 
Leicester rams with Merino ewes. It has been developed into a true type by years of close breeding 
and selection. 
the parents and as uniform as the uncrossed parental lines. The 
second generation, however, will in general show distinctly more 
variability. The ancestral characteristics will be found in every 
compatible combination and in all degrees of development if enough 
young are produced. Characteristics may be found which appear 
wholly new. If a promising new type is formed it remains to fix it 
by careful selection and close breeding. 
There will doubtless always be room for the production of new 
types and from time to time even new breeds. But this work of 
radical improvement is not likely to occupy more than a few of the 
most ambitious breeders. Others will have such superior stock that 
they can do no better than conserve it by close breeding, making such 
slow improvement by selection as the limited variability of the stock 
permits. With a larger number, periods of close breeding must be 
interrupted by periods in which new blood is infused into the stock, 
a certain amount of uniformity being sacrificed to obtain renewed 
