1862.] MUSEUM OF COMPARATIVE ZOOLOGY. 81 



by their larger number, an undue preponderance in the collection of 

 the epoch to which they belong. But there is another advantage in 

 making special faunal collections of fossils ; they suggest comparisons 

 with the faunas of the present time which could not otherwise be made 

 so effectively. Thus far geologists, in identifying the horizons of the 

 successive deposits forming the stratified crust of our globe, have 

 started from the universally accepted assumption that animals , of the 

 same geological age are either identical or closely allied over the most 

 extensive areas. Nothing can be further from the truth than such a 

 view, and we need only to compare the faunas of the present period 

 in remote continents to see how widely these differ. If the remains 

 of past ages, belonging to the same geological periods, have generally 

 appeared to be identical or closely allied, it is chiefly owing to the 

 fact that they have been collected in the same geographical zones ; 

 and at present we find a similar agreement between the living animals 

 of the temperate zone of Europe, Asia, and North America. But 

 when we pass to other zones, the scene is entirely changed ; and so it 

 was in former ages, as we already know from the tertiary mammalia 

 of South America and of Australia, and this, I have no doubt, will be 

 found to be also the case for the older formations, within certain 

 limits, not yet ascertained. The specific differences between the 

 remains of the same age, found in deposits remote from each other, 

 are daily brought out more distinctly ; and since I have begun to com- 

 pare the fossils of America with those of Europe, I am gradually led 

 to infer that no specific identity is likely to be established, finally, be- 

 tween animals which have lived at great distances from one another, 

 even though they were contemporaries. The doctrine of the iden- 

 tity of fossils of the same age will therefore require great modifi- 

 cations. I am already certain that species of the same family, belong- 

 ing to different epochs, but found in corresponding zones of latitude, 

 are frequently much more closely allied than species of the same age 

 belonging to different zones. The time is therefore fast approaching 

 when zoological affinity alone will no longer be a trustworthy criterion 

 of contemporaneity, nor zoological difference, however striking, be taken 

 as evidence of a difference in geological age. This unexpected and 

 probably to many most unwelcome result, I have obtained by a care- 

 ful comparison of many faunas of past ages, arranged in the manner 

 6 



