278 RECENT CYTOLOGY 
of Ancestral Heredity in its original form, are based 
upon a common assumption, which is now shown by 
Mendel’s discovery to have been unfounded. This is 
the assumption that all ancestors of the same degree— 
é.g., grandparents—make a substantially equal con- 
tribution to the hereditary qualities of the offspring. 
Mendel has shown that in the case of particular 
hereditary characteristics this is not the case. 
But if we venture to criticise Weismann’s conception 
in the light of more recent knowledge, it must not be 
forgotten that biology, and especially modern cytology, 
owes a great debt to Weismann. To Weismann is due 
the conception of the isolation of the germ-cells from 
somatic influences, a view which is in complete accord- 
ance with the Mendelian view of the inheritance of 
definite characters. And it was Weismann who first 
emphasized the belief that the chromosomes represent 
those parts of the nucleus which are specially concerned 
in the processes of heredity. These conceptions— 
which, indeed, constitute an essential part of his own 
theory of heredity—have stood the test of time in an 
admirable manner. , 
Let us turn our attention, then, for a short space to 
the Germ Plasm Theory of inheritance. On Weis- 
mann’s theory, as in most other theories of heredity 
from the time of Darwin and Nageli downwards, the 
separate parts of the living organism are supposed to 
be represented by separate material particles in the 
germ-cells. These representative particles are known 
as determinants. A complete set of determinants in 
