1867.] Sir Charles Lyell and Modern Geology. 7 



After all, changes in physical geography and in climate, in any 

 given region, are the chief causes of uncertainty in the application 

 of the percentage test. The climate of Europe, for instance, was 

 much warmer during the Eocene and Miocene periods than it is 

 now. As the climate became colder no doubt the animals and 

 plants which inhabited Europe migrated to warmer regions. In 

 Europe there are many deposits of the age of these warmer periods, 

 and it does not seem unreasonable to believe that formations in more 

 southern latitudes, containing fossilized members of the same fauna, 

 would be more recent in date than the apparently contemporaneous 

 strata in Europe. Indeed, if some members of a species become 

 modified during such a struggle for existence, as takes place in a 

 country whose chmate is becoming unsuitable for its inhabitants, 

 while stronger individuals retain their specific characters; and if 

 the modified form does not survive, as a species, the one from which 

 it descended, it is easy to see that a formation containing a larger 

 proportion of extinct species may be more recent than one containing 

 a smaller proportion, in a different latitude, or possessing in past 

 times a different climate. 



When the percentage test was proposed, the scientific world 

 was not ripe for the consideration of matters so calculated to disturb 

 the principles [of geological chronology, and therefore Sir Charles 

 Ly ell's scheme passed almost unchallenged. That its adoption 

 has been attended with beneficial results is quite certain, and until 

 some better and equally simple scheme is proposed, it will no doubt 

 continue to be the one most generally adopted. But it behoves 

 every philosophical geologist to remember that increase of knowledge 

 has rendered faulty that which at one time appeared to be perfect, 

 "inasmuch as it had the appearance of possessing arithmetical 

 accuracy." * 



As science advances we are rather apt to forget that what to 

 us are mere elementary, and apparently self-evident truths, were 

 at one time original and great discoveries. So the services of our 

 predecessors are not unfrequently too much underrated, and the 

 truth of the old maxim that " familiarity breeds contempt " is 

 proved in a new way. It seems, therefore, a good thing now and 

 then to consider how large a debt we really do owe to those who 

 have gone before us; often men who with imperfect aids have 

 indicated the clue to some of nature's mysteries, which a more 

 perfect knowledge of natural laws now enables us firmly to grasp. 

 And if it should, as no doubt it frequently does, eventually become 

 manifest that old ideas, interpretations, and theories are erroneous, 

 there is not the less credit due to their authors ; for have not their 

 readings of nature for years answered all the requirements of a 

 more perfect interpretation, and materially assisted science thereby 



* 'Quart. Jonin. Geo]. Soc ,' vol. xxii., p. 280. 



