PERIOD OF GESTATION. 75 



It is very difficult to prove the exact date of actual 

 fecundation. The heat-period of the female extends 

 to 9-10 days ; on any one of those days or nights the 

 act of fecundation may take place, and thus the date 

 of birth would be either shortened or lengthened. 



Erxleben (' Systema Eegni Animalis,' 1777) con- 

 siders that the difference is caused by the vpant of 

 knowledge as to when the act of fecundation takes 

 place. 



Domestication may also produce a modification ; 

 it certainly tends to increased fertility. Herbert 

 Spexcer(' Principles of Biology,' vol. ii. pp. 457 cases'.), 

 on the laws of multiplication, nutrition, and genesis, 

 says : — " The best illustrations are those yielded by 

 animals in which we have, besides an increased 

 supply of nutriment, a diminished expenditure. Two 

 classes of comparisons alike in these implications 

 may be made — comparisons between tame and wild 

 animals of the same species or genus, and comparisons 

 between tame animals of the same species differently 

 treated." 



" Equally clear proof," he writes, " that abundant 

 nutriment raises the rate of multiplication occurs in 

 mammals ; compare the litter of a dog (6-14) with 

 the wolf and fox (5-6). Again, the Wild Cat has 

 4 or 5 kittens, while the Tame Cat has 5 or 6 three 

 times a year, no less distinct evidence that among 

 domesticated mammals themselves the well-fed indi- 

 viduals are more prolific than the ill-fed individuals." 



BuFFON states that the Wild Cat, and also the 

 Domestic Cat run wild, only produce their young 

 once a year, whilst the Domestic Cat breeds several 

 times during the same period. 



Bkllixgeki, C. F. (' Delia fecondita degli Animali 



