Page:Popular Science Monthly Volume 41.djvu/838

818 and paleontology renders such explanations almost superfluous. Geology, aided by the deep-sea explorations, has come to a better comprehension of the mechanism of sediments, and it knows what it may expect to find in the rocky archives of the earth, and what it may not; and, on the other side, the discovery of the missing links between past and present has been going on of late with such a rapidity as has outstripped the most sanguine expectations. Our museums already contain whole series of fossil organisms which almost step by step illustrate the slow evolution of large divisions of both animals and plants; our present mammals already have been connected by intermediary forms with many of their Tertiary ancestors; and the paleontologist can already trace the pedigree of birds, and even mammals, as far back as the lizards of the Secondary period—not merely deducing it from embryological data, but by showing the real beings which once breathed and moved about upon earth.

At the same time one point of great moment for the theory of evolution, and only alluded to by Darwin, has been brought into prominence. The part played by migrations in the appearance of new species has been rendered quite obvious. Thus we know perfectly well that the ancestors of our horse migrated over both Americas, Asia, Europe, Africa, and probably back to Asia, and that each step in those migrations was marked, by the apparition of some new characters which are now distinctive of the horse. The same remark applies to the mastodons and their descendants, the elephants; to the common ancestors of the camel and the llama, and to the Ungulata altogether. It may be taken now as a general rule that the evolution of new species chiefly took place when the old ones were compelled to migrate to new abodes, and to stay there for a time in new conditions of climate and general surroundings. The intermediate forms have not been exterminated on the spot; and if we want to obtain the intermediate links between two allied species, the relics of which are found in two geological formations of a given country, we must ransack for fossils all the five continents upon which the intermediate links have been scattered. This is why the discovery of intermediate types has gone on so rapidly since North America, South Africa, South America, New Zealand, and partly Asia began to be thoroughly explored by experienced paleontologists.

Many of the "missing links" were discovered, as is known, in Darwin's lifetime. Thus, the first really bird-like, feathered lizard, the Archæopterix, was unearthed as early as 1862; and eight years later, Prof. O. C. Marsh already described, from the Upper Cretaceous beds of North America, two more lizard-birds, one of which (Hesperornis) must have resembled our present fisheating divers, while the other (Ichthyornis), provided with