86 INTRODUCTION. 



It is, in fact, clear that a pictorial representation of the different 

 groups of the animal kingdom, in the order of their natural alliances, 

 would not exhibit a series of regularly ascending steps, but would 

 have the form of a branched and ramified genealogical tree. Such 

 a tree would exhibit one main stem, which would give origin to nu- 

 merous lateral stems. These latter would, in turn, subdivide, some 

 branches ascending in the course of their development, while others, 

 as the result of degeneration, would descend. 



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

 and " variety," are employed by the palaeontologist in precisely the 

 same sense, and with precisely the same limitations, as by the zoolo- 

 gist. We must notice, however, that a paltzontolog.ical " 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, there- 

 fore, cannot, from the nature of things, be as thoroughly denned as a 

 living one ; and it is both possible and probable that variations in the 

 form of the skeleton, especially if an integumentary one, may often 

 depend upon mere individual, 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 palaeontologists, 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 

 injurious if accompanied with the well-understood reservation that 

 this is done as a matter of convenience only, and that a somewhat 

 wider and looser signification is to be given to the terms " species " 

 and " genus " in palaeontology than would be admissible in zoology. 

 At the same time, this practice may be, and has been, carried too 

 far • and in the case of very variable or " protean " species, it is cer- 

 tainly advisable to adhere to the plan usually adopted by British 

 palaeontologists — namely, to define the species by its central type, 

 and to group the variable forms under this type as varieties. As 

 a general definition, however, of what is understood in palaeon- 

 tology as a species, we may follow Zittel. This distinguished 

 naturalist defines a palaeontological "species" as comprising "all 

 those individuals, or remains of individuals, which possess in com- 

 mon an assemblage of constant characters, and which constitute 

 collectively a distinctly circumscribed morphological series (" For- 



