the gills. The tadpole now rises more frequently to the surface where it
takes small mouthfuls of air. Meanwhile great changes are effected inside
the body where the various systems of fishlike organs become remodeled
into amphibian structures. A sac is formed from the wall of the esophagus,
and this enlarges and divides to form the two simple lungs. The legs
increase in size, the tail dwindles more and more, the gills close up, and
soon the animal hops out on land as a complete young frog. From this time
on it breathes by means of its lungs instead of gills, even though it
returns to the water to escape its foes, to seek its prey, and to
hibernate in the mud of the lake bed during the winter months.

All these changes are familiar and natural, but until science places them
and similar facts in their proper relations their significance is lost to
us. The tadpole is essentially a fish in its general structure and mode of
life, even though its heritage is such that it can develop into a higher
animal. When it does become a frog it proves beyond a doubt that there is
no impassable barrier between fishes and amphibia. Our earlier comparison
of the structures of these two classes of vertebrates led to the
conclusion that the latter had evolved from antecedents like the former,
and had thus followed them upon the earth; now that sequence seems to have
some connection with the method by which a tadpole, obviously not a fish
but nevertheless actually fishlike, changes into a frog, a member of a
higher class of vertebrates. This method is employed by developing frogs
apparently because it follows the ancestral order of events, and because,
so to speak, the only way a frog knows how to become a frog is to develop
from an egg first into a fishlike tadpole and then to alter itself as its
ancestors did during their evolution in the past. We begin to see, then,
that in addition to the impressive fact of development itself, the mode of
organic transformation is far more conclusive evidence of evolution,
because it reveals an order of events which parallels the order