898 J. BARRELL MEASUREMENTS OF GEOLOGIC TIME 



arid climates. But in the following era a new factor entered — a rising 

 oscillating sealevel produced wide recurrent floodings of the lands and 

 led to the deposition of the Eopaleozoic formations. But before that last 

 Precambrian revolution occurred, an era passed away during most of 

 which the lands must have been so flat that the deposits such as those of 

 tlie Animikie were widely spread, and the streams must have meandered 

 with stagnant flow over the low and restricted regions of erosion. Thus 

 tlie Lipaliau or Grand Canyon revolution looms forth in mountainous 

 grandeur between a preceding and a succeeding world of low restricted 

 phiins and still more restricted uplands. 



Lull has seized upon this contrast of physiographic conditions as a 

 factor in the initiation of vertebrate evolution. During the period of 

 peneplanation the sluggish currents which marked the physical environ- 

 ment would not have required sustained movement in floating animals 

 of either the seas or rivers in order than they should remain within their 

 habitat. With the opening, however, of a period of revolution the con- 

 tinents were uplifted, the domain of the fresh waters would expand and 

 they would take on a rapid flow. The freely moving animals of these 

 waters would now have to become vigorous swimmers if they were to 

 stem the currents and survive within their fluviatile environment. Lull 

 regards this environmental stimulus as apparently essential for the de- 

 velopment of the initial fishlike forms of the early chordates, adjusted 

 thus to the flowing fresh waters and leading to the establishment of their 

 dynamic superiority over other phyla. From first to last the chordates 

 have thus found their central field of evolution on the surface of the 

 continents — at first within the flowing waters, later as true terrestrial 

 vertebrates. 



Thus, through the recognition of these wide variations in environmental 

 conditions leading from age to age out of the remote geologic past, sup- 

 planting a picture of monotonous uniformity, the progress of evolution 

 is perceived to have been attained by pulses. Tracing this surge of cause 

 through geologic time and detecting its effects in the progress of organic 

 evolution. Lull has written "the pulse of life."^*^ 



By far the greater portion of the Precambrian consisted of periods of 

 comparative quiet. These, as shown, must have been more than ten times 

 longer than the periods of revolution, but we see them represented chiefly 

 by surfaces of unconformity, whereas the revolution thrusts its existence 

 before the imagination through the evidence of igneous activity and 



i«R. S. Lull: The pulse of life. Chapter IV of The evolution of the earth and its 

 inhabitants. Yale Sigma Xi lectures for 1916-1017; to be published. 



R. S. Lull : Organic evolution, 1917, section .3. Paleontology, pp. 409-691. 



