[Footnote 2: I say nothing of the traces of Birds in the Secondary deposits, because the so-called bird-tracks seem to me of very doubtful character; and it is also my opinion that the remains of a feathered animal recently found in the Solenhofen lithographic limestone, and believed to be a bird by some naturalists, do not belong to a genuine bird, but to one of those synthetic types before alluded to, in which reptilian structure is combined with certain birdlike features.] There was an age in the physical history of the world when great ranges of mountains bound together in everlasting chains the islands which had already grown to continental dimensions,--when wide tracts of land, hitherto insular in character, became soldered into one by the upheaval of Plutonic masses which stretched across them all and riveted them forever with bolts of granite, of porphyry, and of basalt. Thus did the Rocky Mountains and the Andes bind together North and South America; the Pyrenees united Spain to France; the Alps, the Caucasus, and the Himalayas bound Europe to Asia. The class of Mammalia were now at the head of the animal kingdom; huge quadrupeds possessed the earth, and dwelt in forests characterized by plants of a higher order than any preceding ones,--the Beeches, Birches, Maples, Oaks, and Poplars of the Tertiaries. But though the continents had assumed their permanent outlines, extensive tracts of land still remained covered with ocean. Inland seas, sheets of water like the Mediterranean, so unique in our world, were then numerous. Physically speaking, this was the age of continents broken by large inland seas; while in the organic world it was the age of Mammalia among animals, and of extensive Dicotyledonous forests among plants. In a certain sense it was the age of completion,--the one which ushered in the crowning work of creation.