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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