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30 NATURAL SCIENCE [November 



disappear from the records of the rocks ; and practically nothing is 

 known of them until the early Tertiaries, when the various genera 

 and families are almost identical with those surviving at the present 

 day. The only satisfactory specimen of intermediate date is a solitary 

 skeleton (Hylacobatrachus) from the Wealden of Bernissart, Belgium, 

 which seems to represent an animal with persistent gills related to 

 the existing Proteus and Menobranchus. 



The story of the early mammals is almost similar. In rocks 

 dating back to the close of the Palaeozoic and the dawn of the Mesozoic 

 period, there are abundant remains of the Anomodont reptiles or 

 Theromora, which make an extremely close approach in their skeleton 

 to the warm-blooded quadrupeds which we term mammals. They 

 are found in South Africa, India, Bussia, Switzerland, Scotland, North 

 America, and South America. They must thus have been almost 

 world-wide in their distribution. It is also clear that many of them 

 attained a very large size. In all the regions mentioned, however, 

 they completely disappear above the Trias ; and the only known 

 Mesozoic fossils which can be referred to the Mammalia are some 

 fragments of animals no larger than rats from the Jurassic of England 

 and the Jurassic and Cretaceous of North America. It seems, indeed, 

 as if the mammals were evolved in some region of the southern 

 hemisphere which is either now submerged or not yet geologically 

 explored ; for they suddenly appear in great numbers and variety 

 at the base of the Eocene Tertiary both in Europe and North 

 America, which must be the result of migration on the re-arrange- 

 ment of land and sea. It is very curious that notwithstanding the 

 numerous examinations of the Mesozoic and Tertiary strata of Austral- 

 asia and South America during the last half-century, not a single clue to 

 the solution of the problem has hitherto been obtained. As suggested 

 by Mr Lydekker in a recent issue of the Transactions of the Geo- 

 logical Society of South Africa, it is extremely probable that we must 

 turn to a geological exploration of the Dark Continent for the next 

 important advance in our knowledge of the subject. 



Our ignorance of the early land-mammals is strange, but the 

 want of all knowledge of the ancestors of the marine mammals — 

 whales, porpoises, and sea-cows — is still stranger. It is well known 

 that at present all the great Mesozoic marine reptiles of the orders 

 Ichthyosauria, Plesiosauria, and Mosasauria or Pythonomorpha, seem 

 to disappear suddenly at the top of the Cretaceous formations ; while 

 the marine mammals of the orders Cetacea and Sirenia take their 

 place as suddenly towards the top of the Eocene strata. This 

 happens not only in Europe and North America, but also in 

 Australasia, perhaps likewise in South America. Now, we are 

 well acquainted with marine deposits, both of littoral and deep 

 water origin, of intermediate age in many parts of the world. 



