6o PRINCIPLES OF PALEONTOLOGY. 



geological acceptation. Measured by human standards, the 

 majority of existing animals (which are capable of being 

 preserved as fossils) are known to have a high antiquity; and 

 some of them can boast of a pedigree which even the geologist 

 may regard with respect. Not a few of our shell-fish are known 

 to have commenced their existence at some point of the 

 Tertiary period; one Lampshell (Terebratulina caput-serpentis} 

 is believed to have survived since the Chalk; and some of the 

 Foraminifera date, at any rate, from the Carboniferous period. 

 We learn from this the additional fact that our existing animals 

 and plants do not constitute an assemblage of organic forms 

 which were introduced into the world collectively and simul- 

 taneously, but that they commenced their existence at very 

 different periods, some being extremely old, whilst others may 

 be regarded as comparatively recent animals. And this intro- 

 duction of the existing fauna and flora was a slow and gradual 

 process, as shown admirably by the study of the fossil shells 

 of the Tertiary period. Thus, in the earlier Tertiary period, 

 we find about 95 per cent of the known fossil shells to be species 

 that are no longer in existence, the remaining 5 per cent being 

 forms which are known to live in our present seas. In the 

 middle of the Tertiary period we find many more recent and 

 still existing species of shells, and the extinct types are much 

 fewer in number ; and this gradual introduction of forms now 

 living goes on steadily, till, at the close of the Tertiary period, 

 the proportions with which we started may be reversed, as 

 many as 90 or 95 per cent of the fossil shells being forms still 

 alive, while not more than 5 per cent may have disappeared. 



All known animals at the present day may be divided into 

 some five or six primary divisions, which are known technically 

 as " sub-kingdoms. " Each of these sub-kingdoms * may be 

 regarded as representing a certain type or plan of structure, 

 and all the animals comprised in each are merely modified forms 

 of this common type. Not only are all known living animals 

 thus reducible to some five or six fundamental plans of struc- 

 ture, but amongst the vast series of fossil forms no one has 

 yet been found however unlike any existing animal to possess 

 peculiarities which would entitle it to be placed in a new sub- 

 kingdom. All fossil animals, therefore, are capable of being 

 referred to one or other of the primary divisions of the animal 

 kingdom. Many fossil groups have no closely-related group 



* In the Appendix a brief definition is given of the sub-kingdoms, and 

 the chief divisions of each are enumerated. 



