396 BIOLOGY AND ITS MAKERS 
latter being united into ids, idants, etc. The way in which 
he assumes the interactions of these units gives to his theory 
a highly speculative character. The conception of the 
complex organization of the germ-plasm which Weismann 
reached on theoretical grounds is now being established on 
the basis of observation (see Chapter XIV, p. 313). 
The Origin of Variations.—The way in which Weismann 
accounts for the origin of variation among higher animals 
is both ingenious and interesting. In all higher organisms 
the sexes are separate, and the reproduction of their kind is 
a sexual process. The germinal elements involved are seeds 
and pollen, eggs and sperms. In animals the egg bears all 
the hereditary qualities from the maternal side, and the 
sperm those from the paternal side. The intimate mixture 
of these in fertilization gives great possibilities of variations 
arising from the different combinations and permutations of 
the vital units within the germ-plasm. 
This union of two germ-plasms Weismann calls amphi- 
mixis, and for a long time he maintaine] that the purpose 
of sexual reproduction in nature is to give origin to varia- 
tions. Later he extended his idea to include a selection, 
mainly on the basis of nutrition, among the vital elements 
composing the germ-plasm. This is germinal selection, 
which aids in the production of variations. 
In The Evolution Theory, volume II, page 196, he says: 
“Now that I understand these processes more clearly, my 
opinion is that the roots of all heritable variation lie in the 
germ-plasm; and, furthermore, that the determinants are 
continually oscillating hither and thither in response to 
very minute nutritive changes and are readily compelled 
to variation in a definite direction, which may ultimately lead 
to considerable variations in the structure of the species, if 
they are favored by personal selection, or at least if they are 
not suppressed by it as prejudicial.” 
