276 Plctet on the Age of Fossil Groups. 



of fossils, more difficult for those who are not familiar with 

 zoology, it furnishes, on the other hand, the most instructive 

 proof of the successive changes which have occurred at different 

 periods on different parts of the surface of the globe, and it 

 shows how, in the earlier ages, there existed, in different por- 

 tions of the earth, combinations of living beings quite distinct 

 from those which now occupy the same localities, and, at the 

 same time, similar to those which at present exist in other 

 quarters. In proof of this view, I now confine myself to men- 

 tioning the resemblance that exists between some extinct 

 faunas of the Jurassic period, and the actual fauna of Australia. 

 We can trace a similar resemblance between the extinct faunas 

 of other periods, and the living faunas of other parts of the 

 world. On another occasion, for example, I pointed out the 

 resemblance between the fossil floras and faunas of (Eningen 

 and those of the temperate zone of the Atlantic states of North 

 America."* 



Upon this passage M. Pictet pronounces the following 

 comments : — 



" We are quite in accord with M. Agassiz, that an identity 

 of faunas is not in every case a proof that they were contem- 

 porary, and that a difference between faunas does not always 

 prove that they have belonged to a different geological age. 

 But it is not sufficient for us to break up confidence in rules that 

 have been generally admitted ; we must also show in what cases 

 safe conclusions can be reached, and what methods must be 

 pursued. We will commence by considering the case of an 

 identity between two faunas. 



" If two indentical faunas are in each other's neighbourhood, 

 stratigraphy has proved a thousand times that they must have 

 been deposited in the same sea, either by showing that the 

 beds which contain them are continuous, or by demonstrating 

 that they occupy the same place in an analogous series. Nothing, 

 according to us, nor according to M. Agassiz, shakes the gene- 

 rally admitted assertion that identical faunas, situated in the 

 same geographical region, are contemporaneous. 



" If identical faunas are separated by great intervals on the 

 surface of the earth, the question alters, and it may be that 

 identity is no proof of contemporaneity. M. Agassiz, in the 

 citation we have made, speaks of analogous faunas found in dif- 

 ferent ages and at great distances. This singular agreement 

 does not yet rest upon facts sufficiently ascertained, and, with- 

 out wishing to contest its reality, we perceive rather a direction 

 for the future labours of science than an acquisition already 

 made. Such comparisons present great difficulty, for it becomes 



* Not having M. Agassiz's lecture at hand, this passage is retranslated from 

 M. Pictet' 3 article. 



