PERIOD OF GESTATION. 407 



Mr. Darwin states, 1 on the authority of Nathusius, 

 " that merino and Southdown sheep, when both have 

 long been kept under exactly the same conditions, 

 differ in the average period of gestation, as seen in 

 the following table : 



Merinos ,, i i * . 150.3 days. 



Southdowns ..." . . . 144.2 " 



Half-bred merino and Southdown . 146.3 " 

 Three-fourths blood of Southdowns . 145.5 " 



Seven-eighths " " " . 144.2 " 



Tlie average period of gestation in swine is about 

 sixteen weeks. The extremes observed by M. Tessier 

 in twenty-five sows were 109 and 123 days ; 2 and Mr. 

 Fox has reported " ten carefully-recorded cases with 

 well-bred pigs in which the period varied from 101 

 to 116 days. 3 . 



M. Tessier " observes that the extent of gestation 

 is in many species extremely various, and that its pro- 

 longation does not seem to depend upon the age or 

 constitution of the female, or upon the diet, breed, or 

 season, or, in short, upon any known cause." ' 



It is, however, at least probable that the period of 

 gestation is shorter in the breeds that mature early, 

 and this may be the explanation of the difference 



1 "Animals and Plants under Domestication," vol. i., p. 123. 



8 Darwin, " Animals and Plants under Domestication," vol. i., p. 

 95 ; Youatt on " The Hog," p. 154. 



In " The Complete Grazier," p. 299, and in " British Husbandry," 

 vol. ii., p. 511, the extremes observed by M. Tessier are stated at 109 

 and 143 days ; but this is evidently an error. 



3 " Animals and Plants under Domestication," vol. i., p. 95. 



4 "British Husbandry," vol. ii., p. 511 ; "The Complete Grazier," 

 p. 299. 



