196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
a relative one, and relative infertility much greater than this is a well- 
established fact in other cases. Thus, the writer (Castle, 796) 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 indicatious that successful fertili- 
zation depends upon wnlikeness 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 find 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 unz- 
formly the stronger of the two, so that it dominates in every contest, 
for the fertilized egg in such species develops tnvariably 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 approximately 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 im 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 parthenogenetic animals like the honey-bee, each sex uni- 
formly transmits the opposite. May 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 Daphnia, 
