THE BIOLOGICAL RELATIONS OF FOSSILS. 59 
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 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 go 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 ina 
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. 
