538 
responds to such a change of conditions may be seen from 
the experiments of M. Le Sage. It is usual to attribute 
the fleshy character of the leaves of several maritime 
plants to their growth by the seaside where salt can reach 
them ; and that salt is the cause of fleshiness is believed 
from a large number of coincidences, coupled with the 
fact that the same plants are not always fleshy when grow 
ing inland. This hypothesis was confirmed by experiment. 
M. Le Sage succeeded in producing the characteristic fleshi 
ness of maritime plants in inland plants by watering them 
with salt water; and the alteration of the tissues was 
carried on in the second generation from the point gained 
in the first.* 
Another change, of which we have evidence at the 
present time, and which doubtless has taken place during 
the geological ages, is the gradual drying up of lakes. 
“At the north end of Lake Nyasa, old natives will point to a ridge 
of sand ten or more feet in height above the present level of the lake, 
and tell you that they remember the water being at that point ; while 
there is undisputed evidence from observations to show that the 
present average level of the lake is several feet below what it was ten 
or twelve years ago. Lake Shirwa, which may be looked upon as 
neither more nor less than a vast flat basin, has receded on the 
western shore at least a mile, within the memory of lads under 
twenty-five years of age.”—(The Geographical Journal, vol. z'., 
p. 247.) 
The effect of such a change of external condition may be 
seen from certain observations and experiments made by 
Karl Semper, who accidentally observed that the common 
pond-snail (Lymnaea stagnalis) was so remarkably sen 
sitive to the effects of the volume of the water, that in the 
space of six days the difference in the length of those 
living in different volumes of water could be easily and 
* Henslow. Natural Science, vol. i., p. 176. 
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