[Why do animals] undergo in the course of their growth a series of complicated changes, during which they acquire organs which have no function, and which, after remaining visible for a short time, disappear without leaving a trace . . . The explanation of such facts is obvious. The stage when the tadpole breathes by gills is a repetition of the stage when the ancestors of the frog had not advanced in the scale of development beyond a fish.
Thus use your frog: * * * put your hook, I mean the arming wire, through his mouth, and out at his gills, and then with a fine needle and silk sow the upper part of his leg with only one stitch to the arming wire of your hook, or tie the frog's leg above the upper joint to the armed wire; and in so doing use him as though you loved him.