INTRODUCTION. 



series most closely approaches the primitive or ideal form of 

 the sub-kingdom, whilst the highest exhibits the greatest 

 amount of complexity and specialisation of this type. But 

 it is not possible to establish any such linear classification 

 for the animal kingdom as a whole. Given an animal of a 

 lower " sub-kingdom " than another animal, no amount of 

 complexity, no specialisation of organisation, can raise the 

 former above the latter. The one may be the result of the 

 high evolution of a low morphological type, the other may 

 be the result of the low evolution of a higher morphological 

 type, but the superiority of the ground-plan gives the latter 

 the higher place. We must therefore abandon the idea that 

 it is possible to establish a linear classification of the animal 

 kingdom. 



The terms "class," "order," "genus," "sub-genus," "species," 

 and "variety," are employed by the palaeontologist in pre- 

 cisely the same sense, and with precisely the same limita- 

 tions, as by the zoologist. We must notice, however, that a 

 palceontological " species " has not always or necessarily the 

 same value as that which a zoological species ought invariably 

 to possess. This arises from the fact that the determination 

 of fossil species is, almost without exception, based solely upon 

 the characters of the hard parts of the animal these, also, 

 being often but imperfectly preserved. A fossil species, 

 therefore, cannot, from the nature of things, be as thoroughly 

 defined as a living one ; and it is both possible and prob- 

 able that variations in the form of the skeleton, especially 

 if an integumentary one, may often depend upon mere in- 

 dividual, sexual, or local peculiarities, which could be at 

 once discovered in the case of living forms, but which can 

 hardly be detected as regards extinct types. Moreover, 

 there is a practical inconvenience attending the use of the 

 terms " variety " and " sub-genus " in palaeontology, which is 

 not found in zoology, owing to the very different nature of 

 the working material of these two sciences. Many palaeon- 

 tologists, therefore, prefer, as we think rightly, to follow the 

 general practice of giving distinct names to " varieties " and 

 " sub-genera," thus practically raising them to the rank of 

 " species " and " genera ; " and this practice can hardly be in- 



