36 BULLETIN 1151, tJ. S. DEPARTMENT OF AGRICULTURE. 
be practiced, however, only by the most skillful breeders, and by them 
only when they have definite knowledge of ancestry. 
Each breeder will have to decide for himself whether he will 
practice inbreeding with his foxes. Inbreeding tends to fix char- 
acters that can be fixed, but one of its most valuable uses is in bring- 
ing clearly to light the relative merits of different strains in such 
characteristics as fecundity, resistance to disease, and the like, which 
are affected so much by factors other than heredity that they can 
not be fixed in individuals. In these respects progress is more likely 
to result from the selection of strains than of individuals. 
BREEDING AGE. 
Because of the tremendous prices paid for breeding stock and for 
pelts in the early days of the fox industry, little thought was given 
to the important problem of improved breeding. The age at which 
silver foxes should be bred has likewise been given very little atten- 
tion or consideration, the general practice being to select the best 
foxes from the pup crop produced in spring for mating late in fall 
or early in winter. Most of the foxes used for breeding purposes, 
therefore, are first so used at the age of 10 months, and if the first 
season's mating is successful the vixen produces her first litter before 
she is 1 year old. 
Breeding and feeding are now receiving more serious thought. 
A number of breeders believe that better pups are produced if the 
vixen skips the. first season without producing a litter, but just as 
many favor the production of a litter the first year. Some may 
think that they have definitely settled this matter, but among breed- 
ers generally it is a much-debated question. 
Domestic animals as a rule make most of their growth during the 
first year. This is also true of foxes. Intelligent live-stock breeders 
deem it a serious mistake to breed females under a year, for the reason 
that nature has not fitted them to become mothers at this early age. 
Animals born from underaged mothers are generally small and weak, 
and in spite of good care and feeding the mother too often does not 
supply sufficient milk to nourish the offspring. This occurs rather 
commonly among foxes bred the first year, although it can not be 
definitely stated that it is caused by early breeding. If a rancher 
follows the practice of breeding his animals too young he may find 
in a few years that the offspring are degenerating in size, constitution, 
and quality. Objections to early breeding may not apply directly in 
the silver-fox industry, but they are worthy of consideration until 
the facts are definitely established. 
TIME OF BREEDING. 
The mating season occurs principally during the months of Feb- 
ruary and March. The oestrum, or heat period, occurs once a year 
and lasts about three or four days; and while the vixen will accept 
service only at this time it seems to make little difference whether it 
is early or late in the period. Service has been observed to be 
accepted many times during the period of heat, but as a rule not more 
than one service is necessary to insure conception. 
The gestation period is 51 or 52 days, and the young are usually 
born in April and May, Young born between April 15 and May 15 
