72 THE BREEDING OF ANIMALS 



The period of gestation in the domestic jennet is 

 370 days. Careful records kept by Kalo Monsees 1 

 on the large jack- and jennett-breeding farm of Monsees 

 and Sons at Smithton, Missouri, indicates that the maxi- 

 mum gestation period for a living colt was 13 months 

 and 16 days. The minimum period was 11 months and 

 15 days. The average period was 370 days. These 

 figures may be taken as reliable since they cover a large 

 number of animals of varying ages and sizes for several 

 years and represent the progeny of different jacks. 



67. Causes of variation in length of gestation period. 

 The causes of variations in the time required for the 

 development of the young in the uterus are not always 

 clearly apparent. It is probable that prolonged gesta- 

 tion may sometimes be due to the mother suckling young 

 during pregnancy, resulting in providing an insufficient 

 supply of food to the developing fretus. 2 Tesster con- 

 cludes that the length of the period of gestation does 

 not depend upon age, constitution of the female, diet, 

 breed or season. Nathusius 3 and others have found 

 that comparing races and breeds belonging to the same 

 species, those which have been selected for their early 

 maturing qualities have a shorter gestation period. 

 Darwin 4 has given some evidence of this in connection 

 with the .grading up of Merino sheep bred to the earlier 

 maturing Southdown. The length of gestation is given 

 as follows : 



1 Kalo Monsees, "Breeding Records of Monsees and Sons Jack 

 and Jennet t Breeding Farm." 



2 Pinard, "Gestation," Richets Dictionnaire de Physiologie, 

 vol. 7, Paris, 1905. 



3 Loc. cit. ' 



4 Darwin, "Animals and Plants under Domestication," vol. 

 1, p. 123. 



