How slowly and surely the circulatory system improved!  From the cold-blooded animal to the warm-blooded is a great advance.  In the warm-blooded is developed the capacity to maintain a fixed temperature while that of the surrounding medium changes.  The brain and nervous system display the same progressive ascent from the brainless acrania, up through the fishes, batrachia, reptiles, and birds to the top in mammals.  The same with the skeletons in the invertebrates, from membrane to cartilage, from cartilage to bone, so that the primitive cartilage remaining in any part of the skeleton is considered a mark of inferiority.

According to Cope, there has been progressive improvement in the mechanism of the body—­it has become a better and better machine.  The suspension of the lower jaw, so as to bring the teeth nearer the power,—­the masseter and related muscles,—­was a slow evolution and a great advance.  The fin is more primitive than the limb; the limbs themselves display a constantly increasing differentiation of parts from the batrachian to the mammalian.  There was no good ankle joint in early Eocene times.  The model ankle joint is a tongue and groove arrangement, and this is a later evolution.  In Eocene times they were nearly all flat.  The arched foot, too, comes in; this is an advance on the flat foot.  The bones of the palms and soles are not locked until the later Tertiary.  The vertebral column progressed in the same way, from flat to the double curve and the interlocking process, thus securing greatest strength with greatest mobility.  In the earliest life locomotion was diffused, later it became concentrated.  The worm walks with its whole body.

IV

If we figure to ourselves the geologic history of the earth under the symbol of a year of three hundred and sixty-five days, each day a million years, which is probably not far out of the way, then man, the biped, the Homo sapiens, in relation to this immense past, is of to-day, or of this very morning; while the origin of the first vertebrates, the fishes, from which he has arisen, falls nearer the middle of the great year.  Or, dividing this geologic year into four divisions or seasons, primary, secondary, tertiary, and quaternary, the fishes fall in the primary, the reptiles in the secondary, the mammals in the tertiary, and man in the early quaternary.

If the fluid earth hardened, and the seas were formed in the first month of this year, then probably the first beginning of life appeared in the second month, the invertebrate in the third or fourth,—­March or April,—­the vertebrates in May or June, the amphibians in July or August, the reptiles in August or September, the mammals in October or November, and man in December,—­separated from the first beginnings of life by all those millions upon millions of years.