PRINCIPLES OF LIVESTOCK BREEDING. 7 
come in heat from 6 to 8 weeks after calving if suckling the calf, 
otherwise after 3 or 4 weeks. The heat period is of very brief dura- 
tion, lasting only a day or less. There is recurrence every 3 weeks. 
Sows come in heat 3 days after farrowing and again within a week 
after weaning the pigs. After this there is, as in the other cases, 
recurrence about every 3 weeks. Both the duration and the time of 
recurrence seem to vary considerably in different individuals. 
THE GESTATION PERIOD. 
Normally one or more ripe egg cells are released during the heat 
period. If fertilization takes place, there is as a rule no recurrence 
of heat until after the birth of the young. The average length of 
the gestation period is given below for a number of animals. 
Gestation period in mammals. 
Mare 11 months (340 days). 
Jennet 12 months. 
Cow 9* months (283 days). 
Ewe 5 months (150 days). 
Goat 5 months. 
Sow 4 months (114 days). 
Dog 2 months. 
Cat 2 months. 
Guinea pig 2\ months (68 days) . 
Rabbit 1 month (30 days) . 
Rat (22 days). 
Mouse (22 days) . 
The period of incubation in fowls corresponds to the gestation 
period in animals. The following periods are given for comparison: 
Incubation period in fowls. 
Ostrich 42 days. 
Goose 30 days. 
Duck 28 days. 
Turkey 28 days. 
Guinea fowl .28 days. 
Pheasant 23 days. 
Domestic fowl 21 days. 
Pigeon 17 days. 
Canary 13 days. 
The gestation and incubation periods are both subject to consider- 
able variation. There seem to be slight breed differences among the 
domestic animals. Thus, Darwin gives the average period for South- 
down sheep as 144 days compared with 150 days for Merinos. The 
variation among individuals within a breed is, however, more impor- 
tant. Thus, it is about an even chance that a foal will be born 
between 333 and 347 days after service and there is also an even 
chance that it will be born outside of these limits. It is an even 
