150 DOMESTICATED ANIMALS AND PLANTS 
pure with reference to all his characters after they have once 
become entangled with others. 
A second method of improvement. It is very clear that here 
we possess a means of improvement quite different from that of 
simple selection, and, moreover, it is one that will somewhat 
suddenly give rise to new races. The chief difficulty is to find 
and identify the comparatively few individuals that are pure 
with reference to all essential characters, and this is a reason 
for reducing the characters in breeding to the fewest possible. 
As to nonessential characters the new race may remain 
hybrid for all the breeder cares. For example, if he is trying to 
combine amount and quality of milk, he will get along faster if 
he pays no attention to the color of the cows, and selects only 
the few that have the character he is after, leaving the color, for 
the present at least, to behave as a hybrid, to be managed later 
after the high milkers have been isolated. 
Improvement by hybridization complicated. When but one 
nonblending character is involved, a full 50 per cent of the 
offspring of hybrid parents is pure as to that character, it being 
equally proportioned between the two parents. 
If, however, another character be involved, then only a small 
proportion of the offspring that are pure as to the first char- 
acter are pure also as to the second character, and so on for 
additional characters. 
If all the desired characters are recessive, then all that is 
required is to wait until the rare individual appears that has these 
characters and no others ; but if, as in most cases, some of the 
characters sought to be retained are dominant, the separation will 
be a tedious operation. 
Mutation and mutants. Accidental crossing in nature is 
constantly producing new strains, most of which go down in the 
struggle for existence, but some of which are sufficiently vigor- 
ous and prolific to persist. They are seldom equally vigorous or 
equally prolific with the parent strains, else they would long ago 
have developed into good species. These strains can of course 
