SOME PROBLEMS OF REPRODUCTION. 71 
no two such categories exist ; on the contrary, all the evidence 
goes to prove that a gamete of the A brood will pair with one 
of any other brood from B to Z, and so on right through the 
alphabet. Maupas has told us that for two Ciliates in the 
eugamic state to pair successfully they must belong to two 
different cycles of descent—that is, they must be descended 
from two different exconjugates. If we use the term sex for 
such cases as these we must admit the existence of as many 
sexes as there are broods or cycles of the species in existence, 
and that difference of sex means not a binary antithesis of 
characters, but a mere question of kinship, which is a reductio 
ad absurdum. 
We see, then, that exogamy is merely the expression of con- 
sanguineous incompatibility, or allogamy, as it has been long 
termed. So far from indicating latent sex, allogamy may 
or may not coexist with very high binary sexual differentiation. 
In Orchids, for instance, side by side with the majority of 
flowers adapted for cross-fertilisation exclusively, we find one 
or two species that are “ autogamous” or self-pollinating. 
If we call allogamy by the name of “ sex,” it is a sex superim- 
posed on ordinary binary sex, and distinct from it; and the 
question occurs here, in an allogamous species, How many 
sexes are we to ascribe to the innumerable individuals, each 
incapable of self-pollination, but capable of fertilising the flower 
of any other individual ? 
We must remember, too, that in many isogamous forms, even 
those which are exogamous, like Acetabularia, conjugation 
may be multiple, as many as five gametes uniting into the 
single zygote. Admitting the supposition that exogamy in- 
volved latent binary sex, what would be the several functions 
of each of these five gametes? The only conclusion left us is 
the one we have stated, that exogamy expresses not an early 
form of sexuality, but a growing sensibility of the organism to 
the fact that the advantages of karyogamy are not fully gained 
by the union of closely allied gametes ; and this fastidiousness 
we find an increasing factor as we ascend the scale of karyo- 
gamic unions. 
