VIGOR OF oFFspluxo Considerable seasonal variation occurs in the size and vigor of offspring. Thus for exam ple it seems undesirable to breed sheep during the hot weather of summer, even where it is possible as is the case with Dorsets. The unfavorable influ ence of hot weather may be exercised throughout the period of pregnancy but it is doubtless active to some ex tent at the time of impregnation and just previously. Professor Ewart has observed that rabbits fertilized before the eggs are mature are likely to pro duce young which resemble the male, while, if fertilization takes place late in the period of heat, the offspring partake more of the characters of the mother. It is evident, therefore, that the charac ters of the progeny are in some degree related to the condition of the germ cells at the time of fertilization.