INTRODUCTION. XIX 



types." There is one remarkable, highly specialized family of 

 Crossopterygian fishes, the Ccelacanthidae, ranging from the 

 base of the Carboniferous to the top of the Cretaceous with 

 scarcely any modifications which can be regarded even as 

 denoting change in the genera representing 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 (see 

 p. 321). 



Imperfection of the Geological Record. The difficulties in 

 ascertaining and interpreting the facts of Palaeontology 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 accumulation 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 palaeontologist will thus not be 

 surprised to learn, for example, that the Lower Devonian 

 chordate animal Palccospondylus, 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 ArchcKoptei'yx, 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 brick-fields 

 near Grays, Essex. Furthermore, it must be remembered that 



