400 PHILOSOPHY OF ZOOLOGY. 



In every species, except man, there is a particular period 

 of the year in which the reproductive system exercises its 

 energies. Domestication brings on this period earlier than 

 ordinary, and in those animals which have several births in 

 a season, their frequency is considerably increased. In all 

 cases, however, as 1 have formerly mentioned, the season of 

 love and the period of gestation are so admirably arranged, 

 that the young ones are produced at the time wherein the 

 conditions of temperature and food are most suitable to 

 the commencement of life. 



In some animals only one ovum is impregnated, and 

 only one foetus is produced each time. In others, many 

 ova are impregnated at once, and a corresponding number 

 of foetuses are produced at a birth. The circumstances 

 which limit the number of ova each individual is ca- 

 pable of producing, during life, have not been determined. 

 Much depends on the healthy state of the ovaria, and the 

 quantity of nourishment with which they are supplied. 

 When one ovarium was extracted in the sow, HUNTER 

 observed that the number of pigs produced at a farrow, 

 was not diminished, nor the periods between the farrows 

 lengthened ; while she ceased to become pregnant much 

 sooner, and did not produce one-half as many as a perfect 

 sow, with which she was compared *. 



When the sexual union has taken place, and the impreg- 

 nated germ has been detached from the ovarium, it is de- 

 posited in the uterus, for the purpose of being brought to 

 maturity, or at least for being prepared for birth. This 



upon being kept two or more years, according to the same law, are less dis- 

 posed to run to straw, or are more productive of flowers and seeds than 

 those which are sown in the first year. 



* " An experiment to determine the effect of extirpating one ovarium 

 upon the number of young produced." Phil. Trans. 1787, p, 233. 



