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 

 Palaeozoic forms of life, less so in those of the Mesozoic, and 

 still less so in the Tertiary. Each 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. 



