Pictet on the Age of Fossil Groups. 277 



a question of analogies and not of identities, and there is great 

 scope for personal peculiarities of appreciation. M. Agassiz, 

 for example, evidently does not intend to assert that the fauna 

 of (Eningen is identical with that actually living in South 

 America ; he merely wishes to say that there exists, between 

 these two populations, more or less intimate relations, resulting 

 from the identity of certain genera, and an analogy between a 

 jDortion of the species. We do not doubt that researches under- 

 taken under this hypothesis, would furnish new and precious 

 documents. 



"We may, however, while still considering the case of identi- 

 cal faunas, separated by great geographical intervals, look at 

 another side of the question, which has not yet been touched 

 upon by the learned Director of the Cambridge Museum, and 

 which is not directly connected with the arrangement proposed 

 for his collections, but which appear to us to possess great 

 interest. If a series of identical faunas find themselves over a 

 long space parallel to a degree of longitude, it may be that, ac- 

 cording to our view, these resemblances are associated with a 

 series of identical, but not contemporary climates. We will cite 

 an illustration that has been supplied by the study of an interest- 

 ing memoir of M. de Strombeck, in which this geologist shows 

 the parallelism of the cretaceous faunas from Hanover to the 

 middle of France, to which Algeria may be added. A series of 

 identical cretaceous faunas succeed each other throughout this 

 long interval, and we find them well developed in Switzerland, 

 where they form a precious intermediary deposit. At this day 

 Hanover and Algeria have very different faunas, and it is 

 probable that in ancient times the climate of these two regions 

 produced an analogous result. If we consider the two cretace- 

 ous faunas identical in Hanover and Algeria, it is probable that 

 each lived in the two countries when they acquired a mean 

 equal temperature, a circumstance that could not have taken 

 place at the same epoch. We may well conceive the probability 

 that this fauna lived in Hanover at an epoch when the earth 

 was much more highly heated than it afterwards became, and 

 that it always had a tendency to radiate and extend itself. In 

 proportion as the climate changed, and the temperature be- 

 came lower, the individuals that wandered towards the south 

 could continue to exist, while those which journeyed northward 

 would be destroyed. The centre of the fauna has thus been 

 displaced, and by continuing this action it has successively 

 occupied Germany, Switzerland, the basin of the Rhone, Pro- 

 vence, and at last Algeria, where the climate adapted to it 

 arrived at a later time. When this fauna thus arrived at its 

 new southern limits, its northern limits must have been also 

 reached, and when it occupied the south of the geographical area, 



