394 BIOLOGY ANY ITS MAKERS 
animals, even the highest developed, begin in a fertilized egg, 
that structure, minute as it is, must contain all hereditary 
qualities, since it is the only material substance that passes 
from one gencration to another. This hereditary substance 
is the germ-plasm. It is the living, vital substance of organ- 
isms that takes part in the development of new generations. 
Naturalists are agreed on this point, that the more com- 
plex animals and plants have been derived from the simpler 
ones; and, this being accepted, the attention should be fixed 
on the nature of the connection between generations during 
their long line of descent. In the reproduction of single- 
celled organisms, the substance of the entire body is divided 
during the transmission of life, and the problem both of 
heredity and origin is relatively simple. It is clear that in 
these single-celled creatures there is unbroken continuity of 
body-substance from generation to generation. But in the 
higher animals only a minute portion of the organism is 
passed along. 
Weismann points out that the many-celled body was 
gradually produced by evolution; and that in the trans- 
mission of life by the higher animals the continuity is not 
between body-cells and their like, but only between ger- 
minal elements around which in due course new body-cells 
are developed. Thus he regards the body-cells as constitut- 
ing a sort of vehicle within which the germ-cells are carried. 
The germinal clements represent the primordial substance 
around which the body has been developed, and since in all 
the long process of evolution the germinal elements have been 
the only form of connection between different generations, 
they have an unbroken continuity. 
This conception of the continuity of the germ-plasm is 
the foundation of Weismann’s doctrine. As indicated before, 
the general way in which he accounts for heredity is that the 
offspring is like the parent because it is composed of some of 
