“Of course they are toe-nails,” said the Doctor.  “Don’t nails grow on the ends of toes?  All kinds of claws, on the ends of birds’ and other animals’ toes, are the same as nails.  Some are long, sharp, and curved, like a cat’s or a Sparrow’s, and some are flat and blunt, like ours.  I could show you some birds with claws that look just like our finger-nails.  Toes, too, are pretty much the same; only this Sparrow, like most other birds, has but four, with three of them in a line in front, and the other one pointing backward.  That is what makes its foot as good as a hand to hold on with when it perches on slender twigs.  Almost all birds have their toes fixed that way.  Some, that do not perch, have no hind toe; and birds that swim have broad webs stretched between their front toes, like Ducks.  All the different kinds of feet birds have are fitted for the ways they move about on the ground, or water, or among the branches of trees and bushes, just as all their shapes of beaks are fitted for the kind of food they eat and the way they pick it up.  Here are two pictures that will show you several different kinds of feet.  Now you must answer the next question, Nat; what do toes grow on?”

“Feet!” said Nat promptly, then adding:  “But this Sparrow hasn’t any feet except its toes; they grow on its legs, because the rest of the horny part stands up—­I’ve noticed that in Canaries.”

“But all this horny part is the foot, not the leg,” answered the Doctor, “though it does stand 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.”