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having water enough at their disposal, being always sufficiently
provided with aquatic plants, and enjoying heat enough. It can by no
means be said that their evolution was arrested by the cold of winter,
as often happens in mountain ponds, when the cold of autumn sets
in before the tadpoles have achieved their development, so that they
become frogs or toads only in the course of the following year. In
the case of my tadpoles, it seemed that the completion of develop
ment was due to my imprudently feeding them, in the spring of 1891,
on the very substantial flesh of their congeners, and in the course of
some three weeks at most the limbs were evolved, the long tail
disappeared gradually, the very colour and appearance of the skin
underwent considerable changes, and my superannuated tadpoles
became toads at last.”—{Experimental Evolution, p. ///.)
Now these two experiments seem to me to be sufficient
to enable us to explain the rationale of this arrested
development. They indicate two conditions, without which
larval evolution is impossible. In the first place a suffi
cient quantity of food is necessary in order to stimulate
the organism to proceed to the next stage of an inherited
process of development. Only on this hypothesis can we
explain the two-fold fact that, with insufficient food, the
process of development was arrested ; and that through
the agency of the most stimulating food the process was
resumed, so that abundant nourishment led to the ab
sorption of the tail. But there is yet another condition,
without which such evolution is clearly impossible, and
that is, that in those creatures which are born in a larval
state, the circumstances surrounding them should be such
as to render the next stage of their development possible.
Cut off the tadpole from the air of the atmosphere, and it
will not develope into an air-breathing amphibian ; and if
it did it must necessarily perish. Here, then, in the case
of the tadpole of the frog, we have a demonstration of the
conditions under which the animal which breathed the air
of the water by means of gills must have been converted
into the animal which breathed the air of the atmosphere
by means of lungs. But here again we are brought face