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 bemg 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. 
