Reviews — A. Smith Woodward's Vertebrate Pakeontology. 369 



Pliocene to the present day. In like manner the highly specialized 

 rhinoceros, J^lasmotherium (Pleistocene of Eussia), had a very 

 limited range in time and space compared with its more ordinary 

 allies. 



" Persistent Types. — There are a few noteworthy exceptions to the 

 common rule last mentioned, which still await explanation. These 

 are generally referred to as ' persistent types.' There is one 

 remarkable, highly specialized family of Crossopterygian fishes, the 

 Coelacanthidse, ranging from the base of the Carboniferous to the top of 

 the Cretaceous, with scarcely any modification which can be regarded 

 even as denoting change in the genera r(jpresenting it (see p. 78). 

 The case of the Tapirs, ranging practically unaltered from the early 

 Miocene to the present day, is also a striking illustration (p. 321). 



'' Imperfection of the Geological Becord. — The difficulties in 

 ascertaining and interpreting the facts of Palasontology are, of course, 

 greatly enhanced by the imperfection of the geological record on 

 ■which we depend. Every item of knowledge acquired may indeed 

 be literally described as owing to a chapter of accidents. Firstly, 

 the organism must find its way into water where sediment is being 

 deposited, and there escape all the dangers of being eaten ; or it 

 must be accidentally entombed in blown sand or a volcanic accumu- 

 lation on land. Secondly, this sediment, if it eventually happens 

 to enter into the composition of a land area, must escape the all- 

 prevalent denudation (or destruction and removal by atmospheric 

 and aqueous agencies) continually in progress. Thirdly, the skeleton 

 of the buried organism must resist the solvent action of any 

 waters which may percolate through the rock. Lastly, man must 

 accidentally excavate at the precise spot where entombment took 

 place, and someone must be at hand capable of appreciating the 

 fossil and preserving it for study when discovered. Having due 

 regard to the doctrine of Chances, the palseontologist will thus 

 not be surprised to learn, for example, that the Lower Devonian 

 chordate animal Palceospoiidylus, the unique representative of its 

 group at present known, has hitherto been found only in one stratum 

 of flagstone a few inches thick in one quarry in Caithness (see p. 3); 

 that ArchcBopteryx, perhaps the most precious of Jurassic vertebrates, 

 is known only by two specimens and a feather from the Lithographic 

 Stone quarries of Bavaria, which have been worked from time 

 immemorial (see p. 232) ; and that the sole known evidence of 

 a Pleistocene monkey in Britain is a detached molar tooth from one 

 of the brickfields near Grays, Essex. Furthermore, it must be 

 remembered that in every region the series of strata contains 

 only a very discontinuous record of its successive faunas and floras. 

 When a region is a land area, as a rule, no deposit capable of 

 preservation for long periods can accumulate, and the characters of 

 its life can only be inferred from fossils which have been entombed 

 in sediments apparently of the period in question elsewhere. When 

 the region happens to be covered with comparatively deep water, 

 the sediments will contain scarcely any but aquatic organisms, 

 rarely yielding a trace of the life on the nearest land. 



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