EFFECTS OF INBREEDING AND CROSSBREEDING. 
zygosis, are more likely to be deleterious than are their dominant 
allelomorphs. The differentiation among the families is due to the 
chance fixation of different combinations of the factors present in 
the original heterozygous stock. Crossing results in improvement 
because each family in general supplies some dominant factors 
lacking in the others. Dominance or even imperfect dominance in 
each unit character is built up into a pronounced improvement over 
both parent stocks in the complex characters actually observed. 
A certain portion of the increase in vigor of the first cross between 
inbred families is maintained on resuming random mating. One- 
half of this increase is maintained in stock founded on 2 inbred 
lines, two-thirds in the case of 3 lines, three-fourths in the case of 
4 lines, four-fifths in the case of 5 lines and so on. 
It is believed that the results point the way to an important 
application of inbreeding in the improvement of livestock. Nearly 
all of the characteristics dealt with here, like most of those of economic 
importance with livestock, are of a kind which is determined only to 
a slight extent by heredity in the individual. About 70 per cent of 
the individual variation in resistance to tuberculosis and over 90: 
per cent of that in the rate of gain, and size of litter is determined 
by external conditions. Progress by ordinary selection of individuals. 
would thus be very slow or nil. A single unfortunate selection of a 
sire, good as an individual, but inferior in heredity, is likely at any 
time to undo all past progress. On the other hand, by starting a 
large number of inbred lines, important hereditary differences in these 
respects are brought clearly to light and fixed. Crosses among these 
lines ought to give a full recovery of whatever vigor has been lost 
by inbreeding, and particular crosses may safely be expected to show 
a combination of desired characters distinctly superior to the original 
stock. Thus a crossbred stock can be developed which can be main- 
tained at a higher level than the original stock, a level which could 
not have been reached by selection alone. Further improvement 
is to be sought in a repetition of the process—the isolation of new 
inbred strains from the improved crossbred stock, followed ulti- 
mately by crossing and selection of the best crosses for the founda- 
tion of the new stock. 
This method of improvement has not been unknown in the past. 
In fact, most of the recognized breeds of livestock were developed, 
more or less unconsciously, in this way. Close inbreeding was prac- 
ticed by the pioneer breeders—Bakewell, the Collings, Bates, Cruick- 
shank, Hewer, etc. The relatively few promising families and the 
successful nicks between them were the foundation stock of the 
breeds. Further development may be expected by the intelligent. 
application of the same principles. 
