among them sometimes before they go up the
river to spawn, and we want to find out whether they go back to the sea
again, whether they swim directly up the stream, or whether they remain
in the brackish water at the mouth of the river."

"If you don't mind my saying so, what is the use of knowing?" asked
Colin. "I mean, what does it matter as long as the salmon spawns?"

"The salmon is one of the most important food fishes of the country,"
the professor said rebukingly, "and it is as important for us to know
all about its habits as it is to know about the way a grain of wheat
grows."

"I hadn't thought of that," Colin said, a little shamefacedly. "I
suppose everything really is important, no matter how small."

The professor smiled at him.

"If you have much to do with studying fish," he said, "or, indeed, with
any kind of science, you will find out it is always the little things
that tell the story. Take the grain of wheat again. If one kind of wheat
ripens two days earlier on an average than another kind, you might think
that so small a difference wouldn't be of great importance, but those
two days might--and often do--make the difference between a good crop
and one which is frost-bitten and spoiled."

"That's a lot easier to see," agreed the boy. "But, sir," he objected,
"you can pick out one little bit of a field and work on that, and it
will 'stay put.' Fishes wander all over the place."

"We want them to do so, my boy," was the reply.

"How can you work on separate fish? One looks so like another!"

"And for that very reason we're going to tag them."

"Tag them?"

"With a little aluminum button fastened to their tail, just as bad
youngsters fasten a tin can to a dog's tail. Every tag has a number, and
we use aluminum because it corrodes rapidly in salt water."

"Then I should think," said Colin, "that was the very reason why you
shouldn't use it."

"Why not?" asked the professor mildly. "We know that the salmon are not
going to stay in the salt water, because they are going up the river to
spawn. If, therefore, we catch a fish in the nets higher up stream, with
the tag bright and shining, we know that it hasn't been in salt water at
all; if dull and just a little worn away, that the fish with that tag
has been staying in the brackish water near the mouth of the river; but
if it is deeply corroded, that the fish returned to sea for a time. As
you see, a good deal of information is gathered that way.