Page:Popular Science Monthly Volume 76.djvu/589

Rh changes, of movements, of trend and tendency among living things ranging over a period of time equal to many millenniums.

Another life record of especial interest, typical of many scattered up and down the land areas of the globe, is furnished by the Osage division of the Mississippian at Burlington and Keokuk. Of one group of crinoids, the Camerata, these Mississippian limestones have yielded about 250 species, and of other groups a number about equally as great. The beauty and perfection of the individual specimens can be appreciated only by those who have had the good fortune to see the superb collections of Wachsmuth and Springer. Crinoids flourished here in such numbers that beds of limestone 150 feet in thickness are built practically of crinoidal remains and nothing else. The time represented was long enough to allow of a series of modifications of such extent that the crinoid fauna of the Upper Burlington is very distinct from that of the lower beds of the same formation, while the fauna of the Keokuk differs from both. Here again the paleontologist is favored, not only with a wealth of material, but with an opportunity to note the trend and tendency of things. This was the time of greatest development, of highest prosperity, among camerate crinoids. But in the midst of this prosperity the trained paleontologist may discover signs of degeneration, the prophecy of speedy extinction. The law enunciated by Beecher and quoted by Professor Woodward in his address before the geological section of the British Association at its meeting in Winnipeg last summer, is well exemplified in the Mississippian history of this particular group of crinoids. The tendency among any division of skeletal-bearing animals to run to extravagant ornamentation in the way of ribs, nodes, spines or other excesses of dead, useless, skeletal matter, is something that precedes and presages the decline and death of the race. Even in the Upper Burlington the skeleton of the crinoids is heavier than in the Lower; stronger nodes on the plates are produced; more arm plates are incorporated in the dorsal cup; the animals are weighted down with useless matter. This tendency is carried to extremes in the Keokuk limestone, a fact well exemplified by the species figured on plate 15 of Hall's "Geology of Iowa," Volume I., part II. In these species the development of massive spines and heavy nodose plates reaches its maximum. The race has come to the end of its career. When the Keokuk closes, only a few of the simpler forms of the Camerata survive, and even these shortly disappear. The paleontologist sees the operation of the same law, the same trend and tendency, among the Cretaceous Ammonoids; in many other groups of animals it is as clearly manifest; but it would not be profitable, before such a body as this, to carry the discussion farther, even if the limits of the paper permitted. Let me close by