November 15, 1895.] 



SCIENCE. 



671 



ence to the cataolj'smic and supernatural origin 

 ideas of the early geologists and with tributes to 

 Lyell and Darwin as the exponents of uniformi- 

 tarianism and evohition. The present status of 

 thought is defined to be ' that of gradual evolu- 

 tion both of the earth and of organic forms, but 

 not at a uniform rate. ' In other words, we now 

 recognize that the laws and forces of nature 

 work not as along a rigid line but so that the 

 effects are cyclical, and the questions to be 

 solved are how far the changes produced, or 

 some of them have been general, instead of 

 local, and may they or any of them be used to 

 determine the division or subdivision of geologi- 

 cal history for the whole earth ? The adaptation 

 of the European standards of classification were 

 found difficult or impossible for other parts of 

 the world, which led to the idea, against which 

 the author raises a warning voice, that all 

 changes are local. The author claims that there 

 have been ' critical periods ' in the earth's his- 

 tory — physical changes so great as to affect the 

 climates of the earth as a whole and to cause 

 marked changes in organic forms. The physi- 

 cal changes compel organic changes largely by 

 inducing or compelling migration of species, 

 thus bringing about new environments and new 

 struggles for existence between faunas and 

 floras. 



Amongst the signs of such critical periods are 

 mentioned widespread unconformities ; sudden 

 changes in organic forms, affecting not only 

 species but sometimes genera and families ; the 

 introduction of new and higher dominant 

 classes ; the birth of great mountain ranges. Of 

 these the most general and important are con- 

 sidered to be represented by changes in organic 

 forms. This is, therefore, adopted for deter- 

 mining the primary divisions of geologic time. 

 Secondary divisions are based on less general 

 changes and are more local in their application. 



Even the greatest changes are not to be 

 considered as catastrophic. They were not in- 

 stantaneous or simultaneous, but probably ex- 

 tended through hundreds of thousands of years 

 and were propagated gradually from place to 

 place. 



Four such critical periods or revolutions are 

 mentioned. I. The Glacial, in which the physi- 

 cal cause was the great change in climate and 



the attendant oscillations of level, leading to 

 wide migrations and extinctions of organic 

 forms and the appearance of man. II. The 

 Post-Cretaceous, or Rocky Mountain, which re- 

 sulted in the obliteration of the interior creta- 

 ceous sea and the unification of the American 

 continent, during which mammalian life was 

 evolved, and of which the Laramie may be con- 

 sidered as the transition period. III. The Post- 

 Palseozoic, or Appalachian, of which the Permian 

 may be considered the transition period, and 

 which brought about the greatest change in life- 

 forms that is recorded in geologic history. 

 Amongst the physical changes which brought 

 these about are mentioned the evident oscilla- 

 tions of temperature, including even well de- 

 fined and extensive periods of glaciation. IV. 

 The Pre-Cambrian. This is marked principally 

 in the rock system by universal unconformity, 

 as the life system was such an inconspicuous 

 element in those early times. 



We should therefore have practically the divi- 

 sions as now recognized, beginning with the 

 earliest. Eozoic, up to the Cambrian period ; 

 Paleozoic, up to the Triassic, T\dth the Permian 

 as a transition epoch ; Mesozoic, up to the 

 Tertiary, with the Laramie as a transition 

 epoch ; Neozoic, up to the advent of the Ice 

 age, and finally all time from them to the 

 present. 



The author notes that these critical periods 

 have become gradually shorter and shorter, the 

 changes in physical geography less and less, and 

 in consequence the changes in organic forms 

 less and less. The shortest in duration, the 

 least in geographical changes, and the least 

 complete and sweeping in changes of organic 

 forms is the last ; but the effect of the introduc- 

 tion of new dominant types in producing changes 

 has been steadily increasing, as witness the 

 appearance of man. Also the oscillations of 

 temperature at critical periods have increased 

 with time. 



The subject closes with a discussion of the 

 laws of evolution of the organic world and the 

 suddenness of changes and variety of transi- 

 tional forms. As a type in this latter connec- 

 tion is quoted the change which occurred be- 

 tween Paleozoic and Mesozoic times, which is 

 accounted for partly by loss of the record and 



