It was a long, long journey from this formless speck of jelly to the highest grade of life--the mammalian form; and up this long ascent, conducted on a definite plan, life moved without faltering or contradictions. Let us then in our last look at vanished fauna, regard some of the fossil remains of that crowning class of animal life at the head of which stands man--“Time’s noblest offspring and the last.” Our knowledge of the earliest mammals is very incomplete, and is constantly changing, as the discovery of new beds of fossils shows to us new forms, and supplies new information concerning their relationships. The mammals were late in their arrival upon the earth. They could afford to wait, for they had come to stay and to possess the kingdom of nature. The first mammals lived in the first age of the Reptilian time. They were nearly the size of large rats or squirrels, and curiously enough their teeth and jawbones are the portions of their bodies which have survived decay to tell their story. The teeth of mammals differ in construction from the teeth of all other animals, and thus if only one single tooth is found we may be sure that it is a part of a mammalian creature; and, being a mammal, we know certain facts in regard to it, as for instance that it was born alive,[47] and while young was cared for and suckled by its mother. When the teeth are molar, or grinding teeth, we know that the animal lived on grain or herbs--and thus from the formation we may decide that it was a grazing animal; sharp-pointed canine, or dog-like teeth, show a meat-eating animal, a hunting, flesh-seeking creature; worn teeth, teeth with the enamel ground down, tell us that the animal was old. Among the teeth found, some suggest mammals that lived on insects, fruit, and roots, and some teeth found in the ancient rocks are very like those of the kangaroo rat; so that by observing the present animals of this type, we can in general arrive at the habits of these long extinct creatures. Probably the rat and squirrel families, capable of living on roots and nuts, and small enough to hide in crevices, in decayed trees, and under stumps, where the large reptiles could not follow to destroy them, were the earliest mammalian forms. It is a singular fact that the rocks of the Chalk age, the last era of the time before the last world-building period, have afforded no fossil mammals. This is evidently not because there were no mammals in that age of the world; for the rocks of the succeeding periods have yielded abundant forms of many species, showing that mammals had extended, multiplied, and improved exceedingly. Either the circumstances of the close of this age were not favorable to the preservation of such remains, or there are in the chalk rocks fossils which will yet be found.