544 
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