316 



SCIENCE. 



[N. S. Vol. VIII. No. 193. 



These phenomena are not satisfactorily 

 handled by any of the principal theories. 

 They are fairlj' well explained hy the Helm- 

 holtz suggestion of shifting color-curves, 

 nearly as well by the hypothesis of Hering 

 and Hillebrand, that color-sensations possess 

 specific brightening or darkening power, 

 which makes itself more notable as the in- 

 tensity increases. These are but formal 

 explanations, however, and increase rather 

 than diminish the difficulties of the theo- 

 ries to which they are attached. 



4. The theory of von Kries, of different 

 visual mechanisms for bright and faint 

 light, supplements excellently the existing 

 theories, and must be regarded as a distinct 

 step in advance. 



5. A definite and highly probable func- 

 tion has been assigned to the visual purple, 

 the function of adaptation, and of causing 

 or aiding vision in faint light. 



Farther than these at present we can 

 hardly go. The number and variety of 

 known phenomena are great and constantly 

 increasing. Their inter-relations grow 

 every day more complex, and the actual 

 mechanism governing those relations still 

 remains almost entirely unknown. Subjec- 

 tive experiment appears likely to yield 

 little more aid. The various theories have 

 arrived at such a state of perfection, and, 

 thanks to subsidiary hypothesis, to such a 

 state of flexibility, that almost any visual 

 result might probably be explainable by 

 either. Perhaps the most hopeful line of 

 research is that which, like Konig's study 

 of the visual purple, seeks to find some re- 

 lation between color-sensations and phys- 

 ical properties. Since so many phenomena 

 point to photo chemical changes in the eye, 

 it would not be surprising if the next ad- 

 vance should come from the chemical side, 

 rather than from the phj'siological, physical 

 or psychological, which have held the field 

 so long. Frank P. Whitman. 



Adklbekt College. 



A HALF-CENTURY OF EVOLUTION, WITH 

 SPECIAL REFERENCE TO THE EFFECTS 

 OF GEOLOGICAL CHANGES ON ANI- 

 MAL LIFE (III.).* 



4. THE UPPEB CRETACEOUS REVOLUTION. 



Another profound and epoch-making 

 change occurred at the beginning of the 

 Uppei' Cretaceous. In Eurasia, as Kayser 

 states, '' this was one of the greatest changes 

 in the distribution of land and water over 

 almost the whole earth that is known in 

 geographical history. Extensive areas which 

 had for long periods been continents were 

 now overflowed by the sea and covered with 

 Cretaceous deposits;" the Upper Cretaceous 

 strata in certain areas in Germany and Bel- 

 gium resting directly on archean rocks. 

 In America (the Dakota stage) there was 

 also a great subsidence. The Atlantic 

 coastal plain was submerged over what was 

 Triassic soil, also the lowlands from New 

 Jersey through Maryland to Florida, while 

 the Gulf of Mexico extended northward and 

 covered western Tennessee, Kentucky and 

 southern Illinois ; a wide sea connected the 

 Gulf of Mexico with tl'ie Arctic Ocean, and 

 thus the North America of that time was 

 divided into a Pacific and an Atlantic land, 

 the latter comprising the Precambrian and 

 Paleozoic areas. 



As Scott states : " The Appalachian 

 mountains, which had been subjected to 

 the long-continued denudation of Triassic, 

 Jurassic and Lower Cretaceous times, were 

 now reduced nearly to base-level, the Kit- 

 tatinny plain of geographers. The pene- 

 plain was low and fiat, covering the whole 

 Appalachian region, and the only high hills 

 upon it were the mountains of western 

 North Carolina, then much lower than now. 

 Across this low plain the Delaware, Susque- 

 hanna and Potomac must have held very 



* Address of the Vice-President before Section F — 

 Zoology — of the American Association for the Ad- 

 vancement of Science, August, 1898 ; concluded 

 from Science, September 2d. '; - 



