540 
tion, but it is also quite clear that this dwarfed condition 
will be inherited. 
“ Assuming that a young Lymnaea were placed in a lake or stream, 
of which the temperature constantly exceeds the minimum at which 
the snail can begin to grow, during only two months of the year, 
while it never perhaps reaches the high optimum 25°, the mollusc will 
be unable to attain its due proportions during the first year, or to 
grow to its full size even during the second, and thus a dwarfed form 
will inevitably arise. This dwarfed form will still be able to reproduce 
and multiply itself; for the maturation of germinal matter—the ovum 
and sperm—takes place during the winter and early spring, at a time 
when the low temperature of the water hinders all growth, and the 
optimum of warmth for the sexual processes is much lower than that 
for growth. Thus a permanently diminutive race might arise if 
the conditions of temperature above described remained constant for 
several successive years in the lake or stream where the young molluscs 
or the eggs have been deposited. Hence it has been supposed, and 
in many cases no doubt with justice, that the dwarfed races of animals 
which are found on high mountains or in the polar regions—where 
they must meet with the conditions of temperature just described— 
have originated directly from the low temperature hindering their 
growth. This assumption, as is quite evident, perfectly accords with 
my experiments on Lymnaea.”—(Semper. Animal Life. pp. 108-g.) 
It is obvious that any cause which retarded the growth of 
the Lymnaea, until the period of sexual maturity, would 
produce a similar result. In this, as in previous instances, 
we have examples of the direct action of changed con 
ditions, apart from the principle of selection.