But although the animals of the early geological deposits indicate shallow seas by their similarity to our shoal-water animals, it must not be supposed that they are by any means the same. On the contrary, the old shells, crustacea, corals, etc., represent types which have existed in all times with the same essential structural elements, but under different specific forms in the several geological periods. And here it may not be amiss to say something of what are called by naturalists _representative types_. The statement that different sets of animals and plants have characterized the successive epochs is often understood as indicating a difference of another kind than that which distinguishes animals now living in different parts of the world. This is a mistake. There are so-called representative types all over the globe, united to each other by structural relations and separated by specific differences of the same kind as those that unite and separate animals of different geological periods. Take, for instance, mud-flats or sandy shores in the same latitudes of Europe and America; we find living on each, animals of the same structural character and of the same general appearance, but with certain specific differences, as of color, size, external appendages, etc. They represent each other on the two continents. The American wolves, foxes, bears, rabbits, are not the same as the European, but those of one continent are as true to their respective types as those of the other; under a somewhat different aspect they represent the same groups of animals. In certain latitudes, or under conditions of nearer proximity, these differences may be less marked. It is well known that there is a great monotony of type, not only among animals and plants, but in the human races also, throughout the Arctic regions; and some animals characteristic of the high North reappear under such identical forms in the neighborhood of the snow-fields in lofty mountains, that to trace the difference between the ptarmigans, rabbits, and other gnawing animals of the Alps, for instance, and those of the Arctics, is among the most difficult problems of modern science.