Febbuaey 18, 1910] 



SCIENCE 



257 



occurred, especially during periods of pro- 

 nounced continental defonnation, but also 

 during eras of quiescence on lands. Their 

 effect upon sea level is, however, readily 

 confused or masked by local uplift or de- 

 pression and is, therefore, of little value 

 when these become decided. During base- 

 level eras the conditions for recognition are 

 most favorable and the occasional occur- 

 rence of a world-wide ebb constitutes the 

 most exact measure of contemporaneity by 

 which we may correlate. 



The several groups of diastrophic phe- 

 nomena which have been outlined furnish 

 the fundamental basis of classification and 

 correlation. They are obviously very un- 

 equal in scope, character and value. Their 

 geographic range may embrace the sphere, 

 .or fall within the realm of an ocean, or be 

 no more extensive than a mountain dis- 

 trict. Their duration may equal a vast 

 era, or merely a period, an epoch or even 

 only an episode. But all other phenomena 

 are dependent and sequential. Diastro- 

 phism sets the stage and marks off the acts 

 of the earth drama. 



ORGANIC CRITERIA OF CORRELATION 



I turn noAV to those criteria of correla- 

 tion which are most universally employed, 

 the criteria of organic evolution. 



All paleontologists and geologists who 

 are familiar with the geologic side of their 

 science, as distinguished from the biologic, 

 are convinced of the influence of environ- 

 ment on evolution; and they consequently 

 recognize the dependence of species in re- 

 gard to origin, development and distribu- 

 tion, upon the geographic conditions of 

 their period of existence. 



These extrinsic conditions have been by 

 no means uniform from place to place or 

 from time to time; they have varied peri- 

 odically, and their influence upon life has 

 differed in kind and degree according to 



the period. As has been emphasized by 

 Chamberlin, marine life has at certain 

 times been favorably conditioned by ad- 

 mission to broadening epicontinental seas, 

 and alternately unfavorably conditioned 

 by limitation to narrowed habitats on the 

 margins of continental shelves. With sim- 

 ilar effects terrestrial life has ranged over 

 wide and connected continents under 

 genial climates or has been eonflned to 

 provinces of sub-continental or even 

 smaller dimensions. 



Marine life, when favored by extended 

 domain, has also enjoyed genial and largely 

 uniform environment. Shallow, freely 

 communicating waters, traversed by con- 

 tinuous far-circling currents, offered uni- 

 formity. Barriers, whether of lands, or 

 temperature, or sediment, or salinity, did 

 not persist in marked degree at such times. 

 Pressing against the shifting boundaries 

 of his ancestral habitat, the marine mi- 

 grant could advance as the limits receded, 

 no faster, and thus a species fitted to com- 

 pete in the occupation of new territory 

 could spread from the provincial to the 

 cosmopolitan with the corresponding 

 spread of that environment to which it was 

 adapted. 



Assuming that the species persists dur- 

 ing this time with only such variation as 

 might be consistent with identification, the 

 distribution will correspond to the spread 

 of environment. The obvious fact is the 

 presence of the species, or of the fauna, of 

 one locality in another place also. The in- 

 conspicuous, but all-important fact is the 

 control by the geographic factor, which 

 has in any particular case determined a 

 shorter or longer interval of migration. 

 The migrants were descendants, who wan- 

 dered as they could. That they could 

 wander farther than their ancestors was 

 due to a spreading sea, to the sweep of a 

 marine current across a vanishing isthmus 



