8 INTRODUCTORY. [CHAP, 



the later geological epochs originated high up in the northern 

 hemisphere, if not, indeed, in the neighbourhood of the pole itself 

 (which is known to have enjoyed a genial climate during the 

 Tertiary period), and that they gradually migrated southwards in 

 a series of waves, probably under pressure of the development of 

 new and higher types in high latitudes; and it is to such southerly 

 migrations that the present marked differentiation of the fauna of 

 different parts of the earth's surface is chiefly due. Whether such 

 a northerly origin held good for the terrestrial life of the Secondary 

 epoch, there are no means of determining ; but it would appear 

 that the higher animals (which were chiefly reptiles) of that epoch 

 were very similar throughout the world, and that the differentiation 

 of faunas had scarcely, if at all, commenced. Instances of this 

 are afforded, as noticed in the sequel, by the occurrence of an 

 identical genus of mammal (Tritylodori) in the lower Secondary 

 rocks of Europe and South Africa ; as well as by the close alliance 

 between the dinosaurian reptiles from the Jurassic rocks of 

 Europe, North America, Argentina, India, and Madagascar (the 

 genera being in some cases identical), and likewise between 

 the anomodont reptiles of the Trias of Europe and the early 

 Secondary rocks of South Africa and India. 



Reptiles belonging to orders still existing, such as crocodiles 

 and chelonians (tortoises and turtles), had already attained a high 

 degree of development in the Eocene division of the Tertiary 

 period, when many genera now living had already made their 

 appearance, whereas at that time the mammals were quite differ- 

 ent from the modern forms. At the same time the side-necked 

 tortoises (Pleurodira) were the dominant forms in the northern 

 hemisphere, whereas they have now all migrated to southern lands, 

 their place in the north being taken by the more specialised 

 S-necked group (Cryptodira). This, however, is not all, for the 

 rhynchocephalians, of which the sole existing representative is 

 the New Zealand tuatera (Sphenodori), attained their maximum 

 development in the northern hemisphere during the early part of 

 the Secondary epoch, and their southern migration must have 

 taken place during some portion of the same period. The 

 palaeontological history of amphibians is still very imperfectly 

 known, but since the group as a whole is an ancient one, the 



