1892,] The Difficulties in the Heredity Theory. 561 
member from any part; c, regeneration of lost tissues—as 
three steps indicating the gradual but not entire withdrawal 
of the reproductive power into the germ-cells. 
I have not space to consider all the grounds which support 
the view of the separation of the germ-cells in man. Some of 
the more prominent are the very early differentiation of these 
cells in the embryo, observed with a few exceptions in all the 
lower orders of animals, and advancing so rapidly in the 
human female that several months before birth the number of 
primordal ova is estimated at seventy thousand, and is not 
believed to be increased after the age of two and a half years. 
The most patent practical proof is that we may remove every 
portion of the body which is not essential to life and yet the 
power of complete reproduction of a new individual from the 
germ-cells is unimpaired. Among the many reasons advanced 
for pensioning the crippled soldiers of our late war you never 
hear it urged that their children are incapacitated by inherit- 
ance of injuries. The strongest proof, however, rests in the 
evidence I have already cited from heredity of the extraordi- 
nary stability of the germ-cells, which is the safeguard of the 
race. 
2. The specific nature of the germ-plasm must be considered 
before we consider its relations. Wherein lies the conserva- 
tive power of the germ-plasm, and in what direction shall we 
look for its transforming forces? You see at once that mar- 
vellous as is the growth of cells in other tissues, the growth of 
the germ-cell is still more so. 
We find it utterly impossible to form any conception of the 
contents of the microcosmic nucleus of the human fertilized 
ovum, which is less than x of an inch in diameter, but which 
is, nevertheless, capable of producing hundreds of thousands 
of cells like itself, as well as all the unlike cells of the adult 
organism. We can only translate our ideas as to the possible 
contents of this nucleus in the terms of chemistry and 
physics.’ . 
Spencer’ assumed an order of molecules or units of proto- 
'See Ray Lankester, Nature, July 15, 1876. 
- *Principles of Biology, vol. i, p. 256. 
