250 
THEORIES OF HEREDITY. 
Nov., 1889. 
the influence of an external force, or by some unusual exercise 
or practice of a part. Thus I might represent the change 
which occurs in a bone-cell when a bony growth has been 
caused by long-continued pressure ; N might represent the 
change which occurs in a nerve cell when some new habit is 
acquired by long practice. Such altered cells would originate 
correspondingly altered gemmules, indicated by the same 
dark appearance, which would be stored up in the germ-cells, 
and would reproduce similarly altered cells in the offspring. 
I have given a very brief account of the main features of 
the hypothesis of Pangenesis. It is a hypothesis which would 
explain the hereditary transmission of acquired characters. 
At the same time it is beset by difficulties which appear to be 
well-nigh insuperable. 
We will now nroceed to examine another tlieorv of here- 
A. _ V 
ditv, that of Professor Weismann. The theory is called 
“ The Continuity of the Germ-plasm,” the latter name being 
applied to the essential part of the germ-cell which determines 
its development into an individual. The word continuity ” 
is made use of to express the theory that heredity depends upon 
the fact that a minute quantity of this germ-plasm is reserved 
unchanged during the development of the individual, and 
subsequently grows and gives rise to the genn-cells. Hence 
the germ-plasm is continuous from one generation to another 
in an unending succession, and from it the germ-cells of each 
generation are produced. 
The germ-plasm in a germ-cell possesses such a constitu¬ 
tion that, placed under appropriate conditions, an individual 
of a certain species will be produced ; but the germ-cells of 
this individual will also contain the same germ-plasm, and will 
therefore develop into offspring which resemble the parent. 
Parent and offspring resemble each other because both arise 
from the same substance, which develops rather later in the 
case of the offspring. Hence everything which is predeter¬ 
mined in the germ-cell, every blastogenic character, may be 
transmitted, while somatogenic characters cannot be trans¬ 
mitted. 
The theory will be rendered more intelligible if we refer 
to Diagram II., in which the development of a Metazoonlike 
that shown in Diagram I. is represented, according to the 
theory of the continuity of the genn-plasm. Development is 
complete in five stages, the number of the somatic cells being 
doubled in every stage, after their first appearance in the 
second. The first stage (I.) is the fertilised ovum, A, the 
single cell out of which all others are produced. It contains 
germ-plasm from two individuals, the combination being 
