et of its tributary streams. Many ancient
rivers have been utterly changed and some obliterated by general
movements of the earth's crust.

THE STORY OF A POND

Look out of the car window as you cross a flat stretch of new prairie
country, and you see a great many little ponds of water dotting the
green landscape. Forty years ago Iowa was a good place to see ponds of
all shapes and sizes. The copious rainfall of the early spring gathered
in the hollows of the land, and the stiff clay subsoil prevented the
water from soaking quickly into the ground. The ponds might dry away
during the hot, dry summer, leaving a baked clay basin, checked with an
intricate system of cracks. Or if rains were frequent and heavy, they
might keep full to the brim throughout the season.

Tall bulrushes stood around the margins of the largest ponds, and
water-lilies blossomed on the surface during the summer. The bass and
the treble of the spring chorus were made by frogs and toads and little
hylas, all of which resorted to the ponds to lay their eggs, in coiled
ropes or spongy masses, according to their various family traditions. On
many a spring night my zooelogy class and I have visited the squashy
margins of these ponds, and, by the light of a lantern, seen singing
toads and frogs sitting on bare hummocks of grass roots that stood
above the water-line. The throat of each musician was puffed out into a
bag about the size and shape of a small hen's egg; and all were singing
for dear life, and making a din that was almost ear-splitting at close
range. So great was the self-absorption of these singers that we could
approach them, daze them with the light of the lantern, and capture any
number of them with our long-handled nets before they noticed us. But it
was not easy to persuade them to sing in captivity, no matter how many
of the comforts of home we provided in the school aquariums. So, after
some very interesting nature studies, we always carried them back and
liberated them, where they could rejoin their kinsfolk and neighbours.

It was when we were scraping the mud from our rubber boots that we
realized the character of the bottoms of our prairie ponds. The slimy
black deposit was made partly of the clay bottom, but largely of
decaying roots and tops of water plants of various kinds. Whenever it
rained or the wind blew hard, the bottom was stirred enough to make the
water muddy; and on the quietest days a pail of pond water had