658 The American Naturalist. [August, 
- + + Obviously these units cannot become infinitely min- 
ute; however small they may be they must always retain a 
certain size. This follows from the extremely complicated 
structure which we must without any doubt ascribe to them.” 
These units are not, however, ultimate, they are in turn 
extremely complex, and are composed of countless biological 
units of the kind conceived by Niigeli and others. 3. The 
reduction of the chromatin only acquires a meaning when 
taken in connection with the above supposition of distinct 
ancestral plasms, and has no meaning if we accept Hertwig’s 
view that there is a complete fusion of maternal and paternal 
germ-plasm. This meaning is that reduction in the matura- 
tion of germ-cells is sui generis, it does not divide the ances- 
tral plasms into two similar groups, but one daughter-cell 
receives one set of germ-plasms or hereditary predispositions, 
and another daughter-cell receives another; reduction is thus 
differential. According to this view the four sperm and ovum 
daughter-cells would each contain a different set of ancestral 
plasms. 4. The fact that the chromatin substance is doubled 
in the sperm and ovum mother-cells, so that we observe 
double the number of rods characteristic of the species, is to be 
explained as an adaptation to the requirements of natural 
selection, for this doubling and subsequent double division 
render possible an infinite number of combinations (as many, 
in fact, as there are individuals) for Selection to operate upon. 
This explanation of Weismann’s is an example of his apoth- 
eosis of the theory of natural selection. Every process is made 
to suit this theory, which, as we have seen in the first and sec- 
ond lectures, is, in his opinion, the exclusive factor of evolu- 
tion. But this very high degree of mingling and remingling 
of ancestral predispositions would be fatal to evolution, for 
after a combination favorable to survival had been established 
in one generation it would be broken up into a new combina- 
tion, perhaps unfavorable to survival, in the next generation. 
This entire essay upon “Amphimixis,” or the theory of 
mingling of reduced hereditary substance, will, I believe, mark 
a turning-point to decline in Weismann’s influence as a biolo- 
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