Next the number of rutting ewes diminishes. The ram gradually recovers his strength. Whilst constantly employing it with the remaining rutting ewes he again begets male individuals. Now, according to the rule of cross-heredity of sex, the number of females ought to be greatest at the outset, because we here have a male of exceptional force. When the ram is exhausted, according to the theory of cross sexual heredity the males ought to be more numerous. It appears, therefore, that in this experiment of Mayerhofer’s we have something which remains in want of explanation. We shall presently have occasion to speak more fully of the facts which this author communicates respecting the origin of sex in man, and will then return to this subject. The information which we have concerning a stallion (Sir Hercules) belonging to the stud of Count Lehndorff deserves attention. This stallion was twenty-six years old, and had to cover twenty-three mares. The result was twenty-four foals of the male sex. This case of the ardent mares and the old stallion can be explained by the theory of cross-heredity of sex. Particular attention should be here directed to a phenomenon which seems to imply that a sexually exhausted individual always has his advantage of propagating his own sex secured. It is a sort of fulfilment of duty on the part of a strong female animal when her litter shows a majority of male individuals, or male individuals only. In this connection may be mentioned the facts communicated by Settegast, Nathusius-Hundisburg, and De la Tellais, who arrived at their results from experiments upon the proportions of the sexes of the offspring of the domesticated mammalia. * * * * * Attempts to solve the problem of the origin of difference of sex by means of experiments have been numerous. So early as the last century Platz attempted to carry out experiments with living animals. The temperaments of the breeding couple ought to be concordant. Both should be either of warm or cool temperament. Warmth and moisture are primary conditions of fertility, not in plants alone, but amongst the animals also; and (as Mayoor Zsigmond, of Kaschau, proclaimed in 1723) represent the primary conditions of the possibility of a conception.