up, as you say. How could toes grow from legs
without any feet between? They never do! There has to be a foot in
every animal between the toes and the legs. Now what do you call the end
of your foot which is opposite the end on which the toes grow?"

[Illustration: FIG. 1. Ordinary foot of perching birds; 2. Foot of
Nighthawk, with a comb on claw of middle toe; 3. Climbing foot of
Woodpecker, with two hind toes; 4. Grasping foot of Osprey, for holding
prey.]

[Illustration: FIG. 5. Scratching foot of Ruffed Grouse; 6. Wading foot
of Golden Plover, with only three toes; 7. Wading foot of Snipe, with
short hind toe; 8. Wading foot of Green Heron, with long hind toe; 9.
Swimming foot of Coot, with lobed toes; 10. Swimming foot of Canada
Goose, with three toes webbed; 11. Swimming foot of Cormorant, with all
four toes webbed.]

"It's the heel in people, but I should think the hind toe of a bird was
its heel," said Nat doubtfully, and beginning to think he did not
understand.

"You might think so," said the Doctor; "but you would be wrong. All this
horny part that a bird stands up on is its foot. And the top of it,
nearest to the feathers, is the heel. Don't you see, when I bend the
foot _so_," continued the Doctor, as he bent the Sparrow's foot forward,
"that the top of the horny part makes a joint that stands out backward,
in the same position your heel always has? All this slender horny part
of the foot, above the roots of the toes, corresponds to the instep of
your foot, and of course the heel comes next. You must remember the name
of it--the Wise Men call it the _tarsus_."

"Then hasn't a bird got any legs, Uncle Roy, only just feet?" asked
Dodo.

"Oh! yes; legs too, with a knee-joint and a hip-joint, like ours. But
all these parts are up closer to the body, and hidden by the feathers,
so that you cannot see them."

As the Doctor said this there was a great commotion. Quick, who had been
watching the mouse hole all the while, gave a sharp bark and pounced on
something. There was a feeble squeak, and it was all over with a mouse
which had ventured too far from its hole.

"Poor little mousey!" said the Doctor, as he took the limp body from
the terrier's mouth. "It is quite dead. I am sorry, but it might have
nibbled some of my birds. Besides, this is exactly what I wanted to
teach you something about. Who can tell me the difference between a
mouse and a Sparrow?"

"I can!" said Dodo; "it's all difference; a m