It is very difficult to give a generally valid description of the notion of female; defining her as a carrier of ova and the male as a carrier of sperm is insufficient because the relation of organism to gonads is extremely variable; inversely, the differentiation of the gametes does not directly affect the organism as a whole: it was sometimes claimed that as the ovum was bigger, it consumed more living force than the sperm; but the latter is secreted in infinitely greater quantity so that in the two sexes the expenditure balances out.