CHIEF DIVISIONS OF THE ANIMAL KINGDOM. 79 



jurious 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 " genera " 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 certainly advisable to 

 adhere to the plan usually adopted by British palaeontolo- 

 gists namely, to define the species by its central type, and 

 to group the variable forms under this type as varieties. 



The duration in time, or " vertical range," of fossil species 

 varies greatly in different cases. Some species have an 

 extraordinarily extended range, sometimes passing through 

 two or three entire formations, and in such cases they 

 generally exhibit numerous varieties. Others, again, are 

 singularly restricted, and do not pass beyond the limits of 

 a single subdivision of a formation, or sometimes even a 

 single bed. In any case, when a species has once fairly 

 died out, it never reappears again. As a general rule, it is 

 the animals which have the lowest and simplest organisation 

 that have the longest range in time, and the additional pos- 

 session of microscopic or minute dimensions seems also to 

 favour longevity. Thus some of the Foraminifexa appear to 

 have survived, with little or no perceptible alteration, from 

 the Silurian period to the present day ; whereas large and 

 highly -organised animals, though long-lived as individuals, 

 rarely seem to live long specifically, and have, therefore, 

 usually a restricted vertical range. Some genera, as some 

 species, are short - lived ; whereas others extend through a 

 succession of zoological periods with extraordinarily little 

 modification. Among these "persistent types" may be 

 specially mentioned the genus Lingula among the Brachio- 

 poda, and Nautilus among the Cephalopoda, of which the 

 former commenced in the Cambrian and the latter in the 

 Silurian, and both of which are represented by living species. 

 While the great majority' of fossils are extinct, and while 

 many of them are extremely unlike any existing forms, my 

 fossil animal has hitherto been detected which cannot (pe 

 referred to one or other of the existing sub-kingdoms. No 



