PICTET*S PALEONTOLOGY. 49 



This law is true for all classes of animals, but is differently exhi- 

 bited in each, the groups first introduced having, it would seem, un- 

 dergone the smallest amount of change and the converse. Thus, if 

 we compare the mollusca and mammalia, we shall find that the 

 former, which existed at the earliest period, have hardly changed 

 their form since the close of the cretaceous epoch, and that the shells 

 of the tertiary period are generically the same as those now exist- 

 ing; while the mammalia, though only recently introduced, have 

 been subject to many changes, and have required the introduction 

 of new and important groups. 



But this law, though sound as a general expression of results, 

 must not be applied too minutely. The nautilus and the terebratula, 

 the companions of the most ancient form of cephalopodous and 

 brachiopodous mollusca, are still represented by existing species ; 

 and associated with the pachyderms of the earliest tertiary period 

 we find bats and small carnivora, which can hardly be distinguished 

 from animals of the same kind now living.* 



Law 3. " The comparison of the faunas of different epochs shows 

 that the temperature of the earth's surface has undergone change.'' 

 —Page 69. 



This appears from the discovery of the remains of animals in parts 

 of the world where the nearest allied species could not now exist ; 

 and it has also been supposed that the extinct faunas of northern 

 Europe indicate a climate nearly tropical. It is probable however 

 that this law has been the result of a generalization somewhat too 

 hasty, the most that we can safely assert at present being, that there 

 have existed different climates at the same spot at different geologi- 

 cal epochs, and that the changes in the cases we are best acquainted 

 with were sometimes, and most frequently, from a higher tempera- 

 ture to a lower, but sometimes also from a lower to a higher. 



Law 4-. " That the species belonging to the more ancient periods had 

 a wider geographical distribution than the species now living'' — 

 Page 73. 



This is a law rather indicated than demonstrated, and it is clear 

 can only be definitively admitted when the numerous localities 

 still unexamined have given evidence on the subject. There appear 

 to have been certain species now extinct which were formerly com- 

 mon to Europe and America ; while others, and those too of no 

 ancient date, were spread over Europe and northern Asia as far as 

 the frigid zone. This would seem to show that the temperature of 

 the earth was formerly more uniform than it is now ; and it may 

 perhaps also appear that the sea was more shallow than at present, 

 and admitted a wider extension of the marine mollusca. 



* [With regard to the evidence for this and the two succeeding laws, the sub- 

 stance of the author's opinion has bccu given without actual quotation. — Ed.] 



VOL. II.— PART II. E 



