l.E CONTK.] 



Critical Periods. 



that term. It was a field for the exercise of imagination rather than 

 inductive reasoning. 



The basis of modern geology, structural and dynamical, was 

 undoubtedly laid by Lyell in the idea that the study of "causes now 

 in operation" producing structure under our eyes is the only sound 

 basis of reasoning from structure to history; and similarly, the basis 

 of palaeontology was laid by Darwin in the "theory of evolution or 

 origin of organic forms by descent with modifications." But by a 

 natural revulsion from the previous catastrophism, these new views, 

 especially before they were modified by the doctrine of evolution, 

 were undoubtedly pushed much too far, and became embodied in 

 the opposite extreme doctrine of uniformitarianism. According to 

 this view, things have gone on from the beginning at a uniform rate, 

 much as they are going on now — changes of relation of sea and 

 land, now here, now there, now in one direction, now in the other — 

 oscillatory and compensatory, without detectable progress in any 

 direction and without assignable goal. The view was conceived in 

 the spirit of the physicist rather than of the biologist, and may be 

 called physical rather than geological. The many changes in the 

 history of the earth were compensatory not progressive. The under- 

 lying idea was stability rather than evolution. And even Darwinian 

 evolution, when accepted, was supposed to imply evolution at uni- 

 form rate — uniformitarian evolution. 



Now, however, opinions are settling down into a view which is 

 a substantial reconciliation of these two extremes, viz.: that of grad- 

 ual evolution both of the earth and of organic forms, but not at uni- 

 form rate. According to this, as I believe, truer view, in the grad- 

 ual evolution of the earth and its inhabitants as a whole, there have 

 been periods of comparative quiet, during which forces of change 

 were gathering strength, but resisted by an opposite conservative 

 force (crust-rigidity in the case of earth forms, inherited character 

 or type-rigidity in the case of organic forms), and periods of revolu- 

 tion, during which resistance gives way, and conspicuous changes 

 take place with comparative rapidity. Changes indeed go on all the 

 time, but more rapidly at these times. 



I shall not stop to illustrate and show how all evolution, just 

 because it is under the influence of two opposite forces or principles, 



