Life Progressive. 413 
and gradual process. Ninety-five per cent. of the known 
fossil shells in the earliest Tertiary are found to be species 
no longer in existence, the remaining 5 per cent. being forms 
that are known to live in our present seas. In the Middle 
Tertiary, the extinct types are much fewer in number, while 
at the close of the Period the proportion with which we 
started may be reversed, not more than 5 per cent. being 
extinct types. 
All existing animals belong to some five or six primary 
divisions, which are technically known as sub-kingdoms, 
each sub-kingdom to be regarded as representing a certain 
plan of structure, each and every animal embraced therein 
being merely a modified form of this common type. Not 
only are all known living animals reducible to these five or 
six fundamental plans, but also the vast series of fossil forms 
which have come to light in investigations of the earth’s 
strata. While many fossil groups have no closely-related 
group now in existence, but in no case do we meet with a 
fossil animal whose peculiarities do not entitle it to be 
placed in one or other of the grand structural types already 
indicated. The old types differ in many respects from those 
now upon the earth, and the further we go back in time the 
more pronounced does the divergence become. A com- 
parison of the animals that lived in the old Silurian seas 
with those now occupying our oceans, would indicate differ- 
ences so great in many instances as almost to place us in 
another world, this divergence being most marked in the 
Paleozoic forms of life, less so in those of the Mesozoic, and 
still less so in the Tertiary. ach successive formation has 
therefore presented us with animals becoming gradually 
more and more like those now in existence. Though there 
is, however, an immense and striking difference between the 
Silurian animals and those of the present day, yet this differ- 
ence is considerably lessened when a comparison is instituted 
between the Silurian and the Devonian, and this with the 
Carboniferous, and so on down to the present period. 
