174 



SCIENCE 



[N. S. Vol. XXV. No. 631 



do not wonder that not even a call to be 

 the official head of the army of science in 

 this country could induce Professor Os- 

 born to leave the museum v^hieh is at once 

 the monument of his work in the past and 

 the material for his work in the future. 



But, however numerous are the grada- 

 tional forms which have been brought to 

 light, Darwin's principle of the imperfec- 

 tion of the geological record is in no wise 

 superseded. It still remains true that the 

 theory of evolution must stand or fall 

 according to our judgment of the adequacy 

 of that principle of Darwin. If the fossils 

 accessible to observation and collected in 

 our museums afford an approximately 

 complete representation of the life that 

 has existed in past ages, there is certainly 

 no standing ground for any theory of evo- 

 lution. But, while we have seen numerous 

 chasms bridged by series of gradational 

 forms, we have also come to a fuller appre- 

 ciation of the significance of Darwin's 

 principle of the imperfection of the geo- 

 logical record. 



In the conclusion of Darwin's chapter on 

 the subject, he used a striking illustration : 

 "I look at the natural geological record as 

 a history of the world imperfectly kept 

 and written in a changing dialect. Of this 

 history we possess the last volume alone, 

 relating only to two or three countries. Of 

 this volume, only here and there a short 

 chapter has been preserved; and of each 

 page, only here and there a few lines. ' ' In 

 the light of our present knowledge of geo- 

 logical history, we are able to see that even 

 this striking illustration fails to do full 

 justice to the subject. The imperfection 

 of the record consists not merely in the 

 fact that some of the chapters are missing. 

 It appears most strongly when we inquire 

 just what chapters are missing. In the 

 conception of continental history to which 

 I have already referred, we have come to 

 recognize a truth which, in somewhat dis- 



torted form, found expression in the old 

 catastrophism. We have come to recog- 

 nize that comparatively short periods of 

 rapid geographical change alternate with 

 long periods of relative stability or slowly 

 progressive change. This is, in substance, 

 the doctrine of critical periods as formu- 

 lated by Le Conte. It is precisely in those 

 critical periods that the record fails, and 

 the gap is indicated by unconformability. 

 Darwinians and Lamarckians alike must 

 recognize that the periods of rapid geo- 

 graphical change must be the periods of 

 most rapid change in fauna and flora. 

 Evolutionary change must be directly or 

 indirectly the result of a failure of adjust- 

 ment between organism and environment. 

 The doctrine of critical periods has taken 

 somewhat more definite form in Chamber- 

 lin's discussion of the effects of intermit- 

 tent subsidence of the ocean bottom. The 

 critical periods in geological history are the 

 times when the rigidity of the crust yields 

 to accumulating strain, when the ocean bot- 

 tom subsides, when the continents emerge 

 to larger area and higher altitude, when 

 more or less of mountain-making takes 

 place, and when the geographical changes 

 bring in their train the diminution of the 

 atmospheric supply of carbon dioxide and 

 a tendency to cold and arid climates. Then 

 come the long periods in which the conti- 

 nents are slowly denuded, the continental 

 shelves are extended landward by en- 

 croachment of the sea and seaward by sedi- 

 mentation, the quota of carbon dioxide is 

 slowly replenished, and the fauna and flora 

 which had been impoverished gradually 

 expand to their former luxuriance. The 

 chapters which are lost from the record are 

 precisely the chapters which would contain 

 the story of those critical periods, marked 

 by extinction of manifold species, and by 

 rapid change in adjustment to new and 

 more rigorous conditions. The geological 

 record of the progress of life is like a his- 



