There are always gradations in nature, or in natural life; no very abrupt departures. If you find any marked trait or gift in a species you will find hints and suggestions of it, or, as it were, preliminary studies of it, in other allied species. I am not thinking of the law of evolution which binds together the animal life of the globe, but of a kind of overflow in nature which carries any marked endowment or characteristic of a species in lessened force or completion to other surrounding species. Or if looked at from the other way, a progressive series, the idea being more and more fully carried out in each succeeding type--a kind of lateral and secondary evolution. Thus there are progressive series among our song-birds. The brown thrasher is an advance upon the catbird, and the mockingbird is an advance upon the brown thrasher in the same direction. Each one carries the special gift of song or mimicking some stages forward. The same among the larks, through the so-called meadowlark and the shore lark, up to the crowning triumph of the skylark. The nightingale also finishes a series which starts with the hedge warbler and includes the robin redbreast. Our ground-sparrow songs probably reach their highest perfection in the song of the fox sparrow; our finches in that of the purple finch, etc.
The same thing may be observed in other fields. The idea of the flying fish, the fish that leaves the water and takes for a moment to the air, does not seem to have exhausted itself till we reach the walking fish of tropical America, or the tree-climbing fish of India. From the protective coloring of certain insects, animals, and birds, the step is not far to actual mimicry of certain special forms and colors. The naturalists find in Java a spider that exactly copies upon a leaf the form and colors of bird droppings. How many studies of honey-gathering bees did nature make before she achieved her masterpiece in this line in the honey-bee of our hives? The skunk's peculiar weapon of defense is suggested by the mink and the weasel. Is not the beaver the head of the series of gnawers, the loon of divers, the condor of soarers? Always one species that goes beyond any other. Look over a collection of African animals and see how high shouldered they are, how many hints or prophecies of the giraffe there are before the giraffe is reached. After nature had made the common turtle, of course she would not stop till she had made the box tortoise. In him the idea is fully realized. On the body of the porcupine the quills are detached and stuck into the flesh of its enemy on being touched; but nature has not stopped here. With the tail the animal strikes its quills into its assailant. Now if some animal could be found that actually threw its quills, at a distance of several feet, the idea would be still further carried out.