over, an insect at rest is not conditioned as an insect in the air. Let it forsake little by little its aërial life, and rest longer and longer on other bodies. In time it becomes a parasite. The structure it had acquired while in the air becomes useless. The environment being more stable, the opposing actions within are reduced, and the organism lapses into a simpler form. In the insect world we should find the largest number of retrograded species, and so we do. Fleas, bugs, the dream of which sends a shudder through our sleep, creepers in the hair, burrowers in the flesh, form a descending, series, each order carrying with it, in the form of vestiges, reminiscences of a higher state when, as winged insects, its ancestors lived in the open air.
Retrogression of this kind has affected higher orders. An amphibious mammal, taking less to the land and more to the water, would lapse in time into a simpler form. The studies of Prof. Wilder on the embryotic dugong seem to show that dugongs and manatees have descended by retrogression from some ancient hippopotamoid mammal.
Retrogression, whose rationale is not found in our studies on the Eolids, has affected still higher orders. If the elephants of our day are descendants of the mastodons and mammoths which, in Pleistocene days, possessed North America and Europe, as the investigations of Gaudry wellnigh demonstrate; if the living tigers and lions have descended from species whose remains abound in ancient caves, as is probable; if the "grizzly" of the Rocky Mountains is a modified form of the great cave-bear, once so common in Europe, as naturalists believe; if the anthropoid apes of Africa and tropical Asia are survivals from a race which spread beyond the tropics and ranked somewhat nearer to man, as the Mesopithecus of Greece and Dryopithecus of France testify out of Miocene strata, the proboscidians, carnivores, and primates have all suffered retrogression, and, at the advent of man, life having reached its zenith, animal life began a downward curve. If, in the main, the higher has followed the lower, within this cycle of progression the struggle for life would involve another cycle of retrogression. As the savage in presence of civilization often sinks to lower savagery, so a species, outstripped in the race of life, and left hopelessly behind, degenerates, and finally dies.
And as the two cycles, progression and retrogression, are involved in the life-history of the earth, so the two movements may go on simultaneously in the same species. Man himself is such a species. His brain, and its servant, the hand, have attained the utmost development. His digestive system and his foot have been modified but little from a primitive type. Progression above in that which is most distinctively human may involve retrogression below in that which is distinctively animal.
