But there are real “connecting-links"’ between the Fish and the Reptiles.  Passing over the many queer forms which serve as links between the two families, we have but to consider our common frog’s history for a striking example.  The Tadpole has gills, has no limbs, uses its tail like a fish’s fin, eats plants, etc.  Passing through several interesting stages the Tadpole reaches a stage in which it is a frog with a tail—­then it sheds its tail and is a full fledged Frog, with four legs; web-feet; no tail; and feeding on animals.  The Frog is amphibious, that is, able to live on land or in water—­and yet it is compelled to come to the surface of the water for air to supply its lungs.  Some of the amphibious animals possess both lungs and gills, even when matured; but the higher vertebrates living in the water breathe through lungs which are evolved from the air-bladder of fishes, which in turn have been evolved from the primitive gullet of the lower forms.  There are fishes known which are warm-blooded.  Students will kindly remember that the Whale is not a fish, but an aquatic animal—­a mammal, in fact, bringing forth its young alive, and suckling it from its breasts.

So we readily see that it is but a step, and a short step at that, between the land-traveling and climbing fishes and the lower forms of Reptiles.  The Frog shows us the process of evolution between the two families, its life history reproducing the gradual evolution which may have required ages to perfect in the case of the species.  You will remember that the embryo stages of all creatures reproduce the various stages of evolution through which the species has passed—­this is true in Man as well as in the Frog.

We need not tarry long in considering the Reptile family of living forms.  In its varieties of serpents, lizards, crocodiles, turtles, etc., we have studied and observed its forms.  We see the limbless snakes; the lizards with active limbs; the huge, clumsy, slow crocodiles and alligators—­the armor-bearing turtles and tortoises—­all belonging to the one great family of Reptiles, and nearly all of them being degenerate descendants of the mighty Reptile forms of the geological Age of Reptiles, in which flourished the mighty forms of the giant reptiles—­the monsters of land and water.  Amidst the dense vegetation of that pre-historic age, surrounded by the most favorable conditions, these mighty creatures flourished and lived, their fossilized skeleton forms evidencing to us how far their descendants have fallen, owing to less favorable conditions, and the development of other life-forms more in harmony with their changed environment.