256 
THEORIES OF HEREDITY 
Nov., 1889 
The argument thus leads to the conclusion that nearly 
everything which is characteristic of an individual is blasto- 
genic, and therefore can be transmitted by the continuity of 
the germ-plasm. We can thus appreciate Weismann’s 
contention that natural selection, while seeming to decide 
between successful and unsuccessful individuals, is in reality 
deciding between successful and unsuccessful germs. 
Monstrosities can be satisfactorilv explained, in the same 
manner as “ identical twins, by the occurrence of an equi¬ 
valent division instead of a differentiating division of the 
cell which, at some stage of development, is the precursor 
of the doubled part. It has already been shown that when 
the ovum of a frog divides in a certain direction, one cell is 
the precursor of the future front half of the body, the other 
of the future hind half; for if one of them be destroyed the cor- 
responding half is absent. Similarly, if one of them under¬ 
went equivalent instead of differentiating division, a monster 
with two front parts or two hind parts would be produced. 
During the vast succession of differentiating divisions which 
take place in the development of one of the higher animals, 
the cells which represent parts of less and less importance 
are gradually told off. Thus, each finger and toe might be 
represented by a single cell at some period of development; 
and if one of these underwent equivalent divison, such a 
simple monstrosity as the occurrence of a supernumerary 
digit would be accounted for. 
The facts are in reality more complex than appears in this 
description ; for I have only considered the differentiating 
divisions which are concerned with producing the various 
parts of the body, and there are also the other differentiating 
divisions which produce the various groups of cells found in 
each part. Taking these into account also, we see that each 
finger would be represented by a single cell for each of the 
great groups of cells which will take part in its constitution. 
These cells must all undergo equivalent division, for we find 
all the groups of cells represented in each of the two parts 
formed in a double monstrosity. 
It must be remembered that in such a case the fertilised 
ovum possessed such a structure as to predetermine an equi¬ 
valent instead of a differentiating division at that particular 
point. And we know that such monstrosities are in the 
highest degree hereditary, as tiiey would be if the germ-plasm 
were continuous.* 
* See Professor Windle’s interesting papers on Teratology, published 
during tlie last few years in the “Journal of Anatomy and Physio¬ 
logy,” and the “Proceedings of the Birmingham Philosophical 
Society.” 
