196 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



a relative one, and relative infertility much greater than this is a well- 

 established fiict in other cases. Thus, the writer (Castle, '96) showed 

 some years ago that more than 90% of the eggs produced by the 

 hermaphrodite tunicate, Ciona intestinalis, are wholly infertile toward 

 sperm produced by the same individual ; yet toward the sperm of 

 another individual the fertility is almost perfect. This instance is only 

 one of many which might be cited as indications that successful fertili- 

 zation depends upon iinlikeness between the gametes uniting. In the 

 case of the tunicate, which is hermaphrodite, sexual unlikeness between 

 gametes probably does not occur, hence it is some other unlikeness 

 which brings egg and sperm together, and it is not surprising to hud a 

 degree of gametic differentiation between the eggs and sperm of the 

 same individual which is insufficient, in most cases, for successful 

 fertilization. 



On the hypothesis advanced, the zygote must, in all cases, bear both 

 the male and the female characters. In the zygote of a hermaphrodite 

 species, these two characters will exist in the balanced relationship in 

 which they were received from the parents, a relationship which has 

 not been disturbed by segregation, and which accordingly is stable. 

 But in a dioecious species the male and female characters meet anew 

 in a struggle for supremacy at each fertilization. Sometimes one, some- 

 times the other, dominates in the zygote, the vanquished character 

 becoming recessive. Exceptionally, as in the occasional or the mixed 

 hermaphrodite of a dioecious species, the fight is indecisive, and neither 

 combatant is supreme. 



In parthenogenetic species, the female character appears to be uni- 

 formly the stronger of the two, so that it dominates in every contest, 

 for the fertilized egg in such species develops invariably into a female. 

 In dioecious species, on the other hand, neither character, apparently, 

 has any uniform advantage over the other. Males and females are 

 produced in a[)proximatcly equal numbers. In hybridization the con- 

 test between gametes may often be an unequal one, and it will not be 

 surprising to find the gametes of one species uniformly dominant over 

 those of another hi sex as well as in somatic characters. This is a 

 matter to which further attention will presently be given. 



But, it may be objected, the hypothesis presented is improbable 

 because in i)arthenogenetic animals like the honey-bee, each sex uni- 

 formly transmits the opposite. INIay it not be so in dioecious animals 

 also? (See Wedekind, :02.) This suggestion is negatived by the follow- 

 ing considerations : (1) Most parthenogenetic animals, like Daphuia, 



