SOME PROBLEMS OF REPRODUCTION. 77 
of constitutional temperament or vigour characteristic 
only of the race, for— 
(a) It is absent in primitive, agamous types. 
(5) 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 (2) exogamous or (b) sexually 
differentiated. Here the nuclei that fuse to remove this re- 
productive 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 incom- 
patibility 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 reju- 
venescence, 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 ; (2) its chemical composition ; (c) the effects of rest, 
change of form, or change of habit (polymorphism and 
