Hypothetical Continents. 89 



were created on the spot " ? How utterly aimless, then, is 

 palaeontology, reduced in this way to a leviathan catalogue of 

 fossils ! 



It will, therefore, be necessary for me to assume as a 

 postulate the truth of the doctrine of the derivative origin of 

 species • but it is not my intention here to adduce any facts or 

 arguments in its favour, or respecting the causes which have 

 conspired to produce the necessary modification ■ it is enough 

 for my purpose to mention that compulsory wandering, or 

 "migration," as it is termed, is one of them, and the one that 

 most concerns the subject of this article. 



Granting the postulate I have mentioned, the palaeontologist 

 can frequently infer — with greater or less probability of its 

 truth, according to the state of our knowledge on the subject — 

 whence came the progenitors of any particular assemblage of 

 animals and plants. In the case of marine organisms he can 

 also indicate, by reference to the position of geological for- 

 mations of the period, the route by which the ancient ancestors 

 travelled from their own habitation to that of their more 

 recent descendants • in fact, it is chiefly where terrestrial life 

 is concerned that he meets with any serious difficulty, and then 

 generally as to the route. 



During the tertiary period, and probably through all 

 geological time, organic fife was subject to similar laws of 

 distribution as at present, and as climatal, hydrographic, and 

 other physical conditions changed, a corresponding alteration 

 was produced in the faunas and floras of the regions affected. 

 We may therefore consider that migration and emigration of 

 animals and plants were as constantly taking place as elevations 

 and depressions of the land. 



During their wanderings the specific features of organisms 

 became more or less changed ; weak and tender species dying* 

 off or becoming modified, new forms thus coming in, and 

 varieties of old ones being formed, and the representatives of 

 the various faunas becoming entombed at their death in the 

 localities where they had severally existed. Now the paleeon- 

 tologist has to contemplate the results of these changes and 

 migrations ; he has to " try back," and decipher step by step 

 the order of proceeding, and to write a history of the life of 

 each period — not a mere catalogue of names and list of pecu- 

 liarities, like the foreign consular passports, but an intelligent 

 record of the wanderings and fortunes, ancestors and descend- 

 ants, of the different faunas, and even species, with which he 

 has to deal. 



As a few out of many known examples of the evidence of 

 changes which palaeontology has revealed to us, I may cite the 

 correspondence in facies between the recent American flora 



