Some Problems of Reproduction: a Comparative Study of ...

SOME PROBLEMS OF REPRODUCTION. 77
of constitutional temperament or vigour characteristic
only of the race, for— ;
(a) It is absent in primitive, agamous types.
(b) It is slight in groups where parthenogenesis occurs,
though often absolute in closely-allied forms.
(c) It has been lost in apogamous groups.
13. A further evolution of this constitutional weakness takes
place in forms which are either (a) exogamous or {b) sexually
differentiated. Here the nuclei that fuse to remove this reproductive
incapacity by rejuvenescence must be of distinct
origin.
14. Exogamy of isogametes cannot be taken as indicating
latent sex ; it is merely the expression of karyogamic incompatibility
of close blood-relations; this, under the name of
Allogamy, has been long since recognised when associated
with, and superadded to, bisexuality.
15. The constitutional weakness reaches its highest degree
in those organisms where allogamy is most marked; the evil
effects of close-breeding are commensurate with the habitual
advantages of cross-breeding which has here become an
"acquired need."
16. Here again we find, from the occasional existence of
types, strains, or even couples, whose offspring does not
degenerate from close breeding, that the need for allogamy is
not absolute,, but a question of constitutional weakness or
vigour.
17. Since, in all cases of plasmodial and karyogamic rejuvenescence,
we find the migration of the nucleus to foreign
cytoplasm, or the reconstitution of the cytoplasm or of the
nucleus, or a combination of these to be the sole necessary
factors: we infer that the constitutional weakness of the
later terms of a cycle of fission is largely due to the
continuance of the association of nucleus and cytoplast
unchanged.
18. From considerations of (a) the known functions of the
nucleus; (i) its chemical composition; (c) the effects of rest,
change of form, or change of habit (polymorphism and