546 The American Naturalist. [July,. 
the typical form of a very much earlier generation. These 
forces are evidently akin, and in the shades of transition from 
one type to another we would undoubtly find a constant dimi- 
nution numerically in the recurrence of characters of the older 
type, and thus “regression” would pass insensibly into 
“ reversion.” 
Weismann has carried the idea of continuity to its extreme 
in his simple and beautiful theory of heredity, which is 
founded upon the postulate that there is a distinct form of 
protoplasm, with definite chemical and molecular properties, 
set apart as the vehicle of inheritance ; this is the germ-plasm, 
G, quite separate from the protoplasm of the body-cells or 
somatoplasm, S. Congenital characters arising in the germ- 
cells are called blastogenetic, while acquired characters arising 
in the body-cells are somatogenetic. 
To clearly understand this view, let us follow the history of 
the fertilized ovum in the formation of the embryo. . It first 
divides into somatoplasm and germ-plasm (see Diagram III), 
the former supplies all the tissues of the body—n, s, m, d, v, 
nervous, muscular, vascular, digestive, etc—with their quota 
of hereditary structure; the residual germ-plasm is kept dis- 
tinct throughout the early process of embryonic cell division 
until it enters into the formation of the nuclei of the repro- 
ductive cells, the ova or spermatozoa. Here it is isolated 
from changes of function in the somatoplasm, and in common 
with all other protoplasm is capable of unlimited growth by 
cell division without loss or deterioration of its past store of 
hereditary properties ; these properties are lodged in the nucleus 
of each ovum and spermatozoan, and these two cells, although 
widely different in external accessory structure (because they 
have to play an active and passive part in the act of conjuga- 
tion), are exactly the same in their essential molecular struc- 
ture, and the ancestral characters they convey differ only 
because they come along two different lines of descent. When 
these cells unite they carry the germ-plasm into the body of 
another individual. Thus the somatoplasm of each individual 
dies, while the germ-plasm is immortal; it simply shifts its 
abode from one generation to another; it constitutes the 
