310 J. E. S. MOORE. 
late condition, with only half as many chromosomes as they 
had before. 
61. It is conceivable that this capacity of periodically 
altering their chromatic valency, which the cells of both 
animals and plants possess, and which is accompanied in some 
by incipient tail formation, may have arisen in either of two 
ways, Viz. : 
It is conceivable that sexual reproduction may have begun 
before the periodic alteration in chromatic valency was evolved 
at all; but that, owing to the constant doubling of the number 
of the chromosomes after every act of fertilisation, a reduction 
in their number became physiologically necessary ; or it is con- 
ceivable that the periodic variation in chromatic valency was 
evolved first, and that after its introduction, sexual conjugation, 
with all its attendant advantages, became physiologically 
possible. I donot know of the existence of any evidence which 
is decisively in favour of one view or the other. This much, 
however, is certain: on the one hand, variations in the number 
of the chromosomes, of a capricious and indeterminate character, 
are known to exist to-day, as in the case of the cellular struc- 
tures accessory to the reproduction of many plants ; while, on 
the other hand, variations which are neither capricious nor 
indeterminate exist, as we have seen, at certain times and 
places among the elements of complex animal and vegetable 
forms ;—and I think most people will probably be disposed to 
agree in regarding this orderly variation as the expression of 
adaptive selection, which has worked towards some physiological 
end in the past, and the disorderly variation, if I may use the 
term, as the remains of something approaching a primitive 
chaos, on which adaptive selection has not yet acted. 
I regard the above speculations, however, of relatively small 
importance beside the long series of structural homologies 
which I have established before, during, and after the synaptic 
phase in the reproductive cycles of both animals and plants, 
because this close correspondence, among a host of complex 
structural details, renders it improbable in the extreme that 
the two series of phenomena can have been independently 
