GEOLOOIOAL CIIANrxE, AND TIME. 119 



As it is now, so must it have been in jtast time. Hntton and Play- 

 fair pointed to the stratified roclis of tlie eartlTs crust as demonstra- 

 tions that the same ])rocesses wliicli are at work to-day have been in 

 oi)eration from a remote antiquity. By thus phicing tlieir theory on a 

 basis of actual observation, and providing in the study of existing 

 oj^erations a guide to tlie interi)retation of those in past times, they 

 rescued the investigation of the history of the eaith from the specula- 

 tions of theologians and cosmologists, and establislied a place for it 

 among the recoginzed inductive sciences. To the guiding influence of 

 their i»hilosophical system the i)rodigious strides made by modern 

 geology are in large nu^asure to be attributed. And here in their own 

 city, after the lai)se of a hundred years, let us oj'fcr to tlu'ir memory 

 the grateful homage of all who have ]trotited by tlieir labors. 



But while we recogidze with admiration the far-rcacliing intluenceof 

 the doctrine of uniformity of causation in the investigation of the his- 

 tory of the earth, we must upon reflection admit that tlw doctrine has 

 been pushed to an extreme jterhaps not contemi»lated by its original 

 founders. To take the existing conditions of Nature as a platform of 

 actual knowledge from which to start in, an inquiry into former condi- 

 tions was logical and prudent. Obviously, however, human experience, 

 in the few centuries during which attention has been turned to such 

 subjects, lias been too brief to warrant any dogmatic assumption that 

 the various natural processes must have been carried on in the past 

 with the sanu' energy and at the same rate as they are carried on now. 

 Variations in energy might have been legitimately conceded as possi- 

 ble, though not to be allowed without reasonable proof in their favor. 

 It was right to refuse to admit the operation of s])eculative causes of 

 change when the phenomena were ca]»able ol" natural and adequate 

 explanation by reference to causes that can be watched and investi- 

 gated. But it was an eri-or to take for granted that no other kind of 

 process or intiiu'ni-e, nor any vaiiation in therateof activity save those 

 of which man has had actual cognizance, has j)layed a part in the ter- 

 restrial economy. The uniformitarian writers laid themselves open to 

 the charge of maintaining a kind of perjx'tual motion in the machinery 

 of Nature. Tiiey could iind in the records of the earth's history no 

 evidence of a beginidng. no prospect of an end. They saw that many 

 successive renovations and <lestructions had been efle<'ted on the 

 earth's surface, and that this long line of vicissitudes ibrmed a series 

 of which the earliest were lost in anti(]uity, while the latest were stdl 

 in ])rogress towards an apjuirently illimitable future. 



The (discoveries of WiUiam Smith, had they been adequately under- 

 stood, would hav(^ been seen to olfer a corrective to this rigidly uni- 

 formitarian conception, for they revealed that the crust of the earth 

 contains the long record of an unmistakable older of progression in 

 organic types. They provc<l that ])hints and animals have varied 

 widely in successive periods ol" the earth's history: the in-eseut con- 



