6.  The change in organisms throughout geologic time has been A progressive change.  In the earliest ages the only animals and plants on the earth were lowly forms, simple and generalized in structure; while succeeding ages have been characterized by the introduction of types more and more specialized and complex, and therefore of higher rank in the scale of being.  Thus the Algonkian contains the remains of only the humblest forms of the invertebrates.  In the Cambrian, Ordovician, and Silurian the invertebrates were represented in all their subkingdoms by a varied fauna.  In the Devonian, fishes—­the lowest of the vertebrates—­became abundant.  Amphibians made their entry on the stage in the Carboniferous, and reptiles came to rule the world in the Mesozoic.  Mammals culminated in the Tertiary in strange forms which became more and more like those of the present as the long ages of that era rolled on; and latest of all appeared the noblest product of the creative process, man.

Just as growth is characteristic of the individual life, so gradual, progressive change, or evolution, has characterized the history of life upon the planet.  The evolution of the organic kingdom from its primitive germinal forms to the complex and highly organized fauna-flora of to-day may be compared to the growth of some noble oak as it rises from the acorn, spreading loftier and more widely extended branches as it grows.

7.  While higher and still higher types have continually been evolved, until man, the highest of all, appeared, the lower and earlier types have generally persisted. Some which reached their culmination early in the history of the earth have since changed only in slight adjustments to a changing environment.  Thus the brachiopods, a type of shellfish, have made no progress since the Paleozoic, and some of their earliest known genera are represented by living forms hardly to be distinguished from their ancient ancestors.  The lowest and earliest branches of the tree of life have risen to no higher levels since they reached their climax of development long ago.

8.  A strange parallel has been found to exist between the evolution of organisms and the development of the individual.  In the embryonic stages of its growth the individual passes swiftly through the successive stages through which its ancestors evolved during the millions of years of geologic time.  The development of the individual recapitulates the evolution of the race.

The frog is a typical amphibian.  As a tadpole it passes through a stage identical in several well-known features with the maturity of fishes; as, for example, its aquatic life, the tail by which it swims, and the gills through which it breathes.  It is a fair inference that the tadpole stage in the life history of the frog represents a stage in the evolution of its kind,—­that the Amphibia are derived from fishlike ancestral forms.  This inference is amply confirmed in the geological record; fishes appeared before Amphibia and were connected with them by transitional forms.