Page:EB1922 - Volume 32.djvu/27

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TABLE I. PROGRESS IN PALAEONTOLOGY.

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TABLE I. Life Epochs and Geologic T.me Units of Europe and North America (After Pirsson and Schuchert, 1915; Usued by Osborn in 1918)

The Cambrian fauna has been made known to us in large measure through the field discoveries and monographic studies of Philip Lake (1906) for Great Britain, of Walcott (1909-21) for North America, and of Cowper Reed (1915) for India. The great variety and high specialization of the Cambrian marine forms, including representatives of all the known marine in- vertebrate phyla, is in harmony with the trend of discovery among the vertebrates, which is to put the origin of existing families very far back into the Age of Mammals and even into the Age of Reptiles (Mesozoic). In fact, the antiquity and per- sistence of modern types, as distinguished from modern genera and species, is an illustration of a very far-reaching principle, namely, that the most stable form of energy in matter known is that of the heredity chromatin on which this extraordinary pres- ervation of the main features of the ancestral type depends. Next to the stability of the properties of the chemical elements, which are now known to pass into each other by transmutation, the most stable physicochemical properties are those which form the heredity basis of life.

Freshwater and Terrestrial Origins. The eurypterids appear as contemporaries of the Cambrian trilobites and traces of them are found in Precambrian rocks; they attain to their acme in Silurian time and develop into the eight-foot giants of the fauna of the Devonian of Scotland and eastern North America, suffering extinction at the close of the Palaeozoic. In 1916 appeared Marjorie O'Connell's memoir, entitled The Habitat of the Enryptcrida, giving as the summation of her studies that through- out their entire phylogenetic history the eurypterids lived in the rivers, a conclusion accepted in the main by Schuchert (1916), with the modification that they also appeared to have lived at times in the brackish waters of more or less large bays and possi- bly in limited numbers even in the seas. Many other origins formerly traced to the sea have more recently been traced to fresh water. T. C. Chamberlin (1900) proposed the hypothesis of a prevailing freshwater origin both for the ancestral backboned animals known as chordates as well as for the much more ancient arthropods, the eurypterids. His strong influence was needed to overcome the widespread notion that all forms of life originated in the sea; and, one after another, theories of freshwater and terrestrial origin have replaced the theory of marine origin. Early in 1916 Barrell pointed out the influence of Silurian- Devonian climates on the rise of air-breathing vertebrates and freshwater origin in Devonian time under seasonal rainfall.

Schuchert continues that the probable freshwater life of the eurypterids opens a vista into continental life as far back as the Upper Cambrian. Other merostomes related to the eurypterids radiated out from the fluviatile faunas of Cambrian, Ordovician and Silurian time, while in the Devonian rivers dwelt great spider-like eurypterids together with forms so similar to scor- pions that they might be called river scorpions, and others that were active swimmers. O'Connell's argument regarding the freshwater eurypterids applies equally to Limulus, the horse- shoe crab. In brief, the existence of freshwater faunas no less varied than the marine faunas is beginning to be traced back to Lower Cambrian time. O'Connell shows that the entire phylogeny of the eurypterids, which includes about 160 species from the Precambrian to the end of the Palaeozoic, dis- tributed in 78 geologic horizons throughout the world, points to migrations like those of fishes from the headwaters of inter- lacing river systems, and, taken with other evidence, strongly supports the theory of Predevonian river life as opposed to the general assumption of marine life of all early faunas.

It now appears that beginning in Precambrian time the trilobites, by wide adaptive radiation, reached the acme of their development in the Cambrian, displaying a high degree of articu- lation and specialization of appendages, suffered a marked decline after the Silurian, and became extinct at the end of the Palaeozoic. James Perrin Smith, who has made a very ex- haustive analysis of cephalopod evolution and especially of the Triassic ammonites, observes that the evolution of form con- tinues uninterruptedly even where there is no evidence whatever of environmental change.