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But in spite of these strong assertions, we may doubt 
whether the doctrine of the continuity of the germ plasm, 
as expounded by Dr. Weismann, does give that support 
to Natural Selection which it is asserted to have done. 
And this for two reasons. First, because the teachings 
of Weismann himself on the subject of the immutability 
of the germ plasm are not consistent; and, secondly, 
because it would be extremely difficult to prove that the 
germ plasm underwent modification only in connection 
with sexual reproduction. 
The teachings of Dr. Weismann are not self-consistent; 
for in one place he tells us that the stability of species 
is an argument for the absolute unchangeableness of the 
germ plasm. But the germ plasm cannot be absolutely 
unchangeable and at the same time modified by every 
act of sexual reproduction. Nor does the stability of 
species necessarily imply that the germ plasm is alto 
gether unmodified ; for stability of species permits, as we 
have pointed out, a very considerable amount of modi 
fication in individuals, and it is the result, not of the total 
absence of variety, but rather of the fact that no kind of 
selection has been applied. 
On the other hand, Dr. Weismann has admitted, in 
certain passages, that the germ plasm may have been 
modified by external circumstances. 
“ It seems to me the possibility is not to be rejected, that in 
fluences continued for a long time, that is, for generations, such 
as temperature, kind of nourishment, etc., which may affect the 
germ-cells as well as any other part of the organism, may produce 
a change in the constitution of the germ-plasm. But such influences 
would not then produce individual variations, but would necessarily 
modify in the same way all the individuals of a species living in 
a certain district. It is possible, though it cannot be proved, that 
many climatic varieties have arisen in this manner. Possibly other 
phenomena of variation must be referred to a variation in the struc 
ture of the germ-plasm produced directly by external influences.” 
—{Apud Eimer. Organic Evolution.' p. 174.) 
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