274 Proceedings of the Royal Physical Society. 
divisions, all the daughter-nuclei disintegrate except two 
in each unit; these copulatory nuclei may be termed male 
and female. The male copulatory nucleus leaves the animal 
in which it arose, passes to the other conjugate, and there 
unites with the female copulatory nucleus. 
This process Weismann interprets as amphimixis—the 
mingling of the hereditary idioplasms of two individuals. 
The micronucleus of Infusoria possesses nuclear rods or 
idants; among these there may be individual differences ; 
before conjugation there is a preliminary enlargement, which 
Weismann assumes to be connected with a doubling of the 
idants by longitudinal division; the first and second divisions 
are “reducing divisions,” the third is an “equal division” ; 
the result is a fresh grouping of the idants, just as in the 
analogous “reducing divisions” of the egg and sperm. 
“Variety of individual character in the hereditary substance 
is thus brought about by means of these divisions.” 
The two conjugating nuclei are essentially similar, like the 
nuclei of Metazoan sex-cells, according to Weismann, Stras- 
burger, and O. Hertwig. The import of their fusion is no 
rejuvenescence or vitalisation, but a mingling of hereditary 
tendencies. And as Weismann now holds that “a belief 
in the inheritance of acquired characters by the highly 
differentiated Protozoa, as well as by Metazoa, must be 
opposed,” and imagines that “the phyletic modifications of 
Protozoa arise from the germ-plasm, that is from the 
idioplasm of the nucleus,” he “can understand why nature 
has laid so much stress on the periodical mingling of the 
nuclear substance of two individuals—why she has introduced 
amphimixis among these animals.” “Clearly it has arisen 
from the necessity of providing the process of natural 
selection with a continually changing material, by the com- 
binations of individual characters.” 
So, in conclusion, the deeper significance of every form of 
amphimixis consists in the creation of hereditary variability. 
Nature has given it the widest possible range by rendering 
unicellular germs incapable of developing alone. And as 
in nature “the useful becomes necessary as soon as it is 
