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 comparison of the animals that lived in the old Silurian seas with those now occupying our oceans, would indicate differences so great in many instances as almost to place us in another world, this divergence being most marked in the Palæozoic 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 difference 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.