Nov., 1889. 
THEORIES OF HEREDITY. 
247 
would then resemble each other, because they are developed 
from the same thing, although at different times. 
There is an essential difference between these two theories 
of heredity. In the first, the germ-cells may bear the impress 
of every event which happens to the somatic cells during the 
life of the parent, and such characters may therefore be looked 
for in the offspring; in the second, offspring and parent 
can only resemble each other in characters which were 
predetermined in the germ-cell from which the parent 
arose. These latter characters, the peculiarities of any 
somatic cell which follow from the structure of the original 
germ-cell, have therefore been called blastogenic by Weismann. 
They have also been called spontaneous , because they spring 
up in the individual without reference to the causes which 
operate during its lifetime; also inherent or centrifugal , because 
they belong to the essential nature of the individual, and 
because they may be looked upon as developing from within 
rather than as impressed from without. Conversely, the 
characters which appear in the somatic cells as the result of 
external influences, or as the outcome of their own special 
or unusual activities,—in fact, any characters appearing in the 
body which were not predetermined in the original germ-cell, 
have been called somatogenic , because their origin cannot be 
traced to the structure of the original germ-cell, but is 
entirely due to events brought about in somatic cells; they 
are also called acquired, because the individual comes to possess 
them, although they do not belong to its essential nature; and 
centripetal, because they are impressed upon the individual 
from without, and are not the outcome of internal causes. 
It is my object to give a more detailed account of these 
two theories of heredity, and then to very briefly allude to some 
of the evidence which is believed to establish the hereditary 
transmission of acquired or somatogenic characters. 
The first theory, maintaining that a close relationship of 
a material kind exists throughout life between somatic and 
germ-cells, was suggested by Darwin, under the name of 
Pangenesis. 
This theory is illustrated by Diagram I., in which the 
large circles, indicated by the capital letters H toO, represent 
the somatic cells of a Metazoon, which, for the sake of 
simplicity, is supposed to be composed of only sixteen somatic 
and four germ-cells, the latter being placed in the centre. 
The somatic cells are arranged in pairs, H H, II, &c., in 
order to indicate the fact that similar cells are generally 
found on opposite sides of the body in the higher Metazoa 
(bilateral symmetry). 
