THE BIOLOGICAL RELATIONS OF FOSSILS. 59 



caput-serpentis] is believed to have survived since the Chalk ; 

 and some of the Forarninifera 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 simultaneously, but that they com- 

 menced their existence at very different periods, some being 

 extremely old, whilst others may be regarded as compara- 

 tively recent animals. And this introduction 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 Ter- 

 tiary 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 now in existence ; but in no case do we meet 

 with any grand structural type which has not survived to the 

 present day. 



The old types of life differ in many respects from those now 

 upon the earth ; and the further back we pass in time, the 

 more marked does this divergence become. Thus, if we were 

 to compare the animals which lived in the Silurian seas with 



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

 the chief divisions of each are enumerated. 



