the remains of quadrupeds corresponding to our
ruminating animals, we infer not only land, but grassy meadows also, and
an extensive vegetation; where we find none but marine animals, we know
the ocean must have covered the earth; the remains of large reptiles,
representing, though in gigantic size, the half aquatic, half
terrestrial reptiles of our own period, indicate to us the existence
of spreading marshes still soaked by the retreating waters; while the
traces of such animals as live now in sand and shoal waters, or in mud,
speak to us of shelving sandy beaches and of mud-flats. The eye of the
Trilobite tells us that the sun shone on the old beach where he
lived; for there is nothing in Nature without a purpose, and when so
complicated an organ was made to receive the light, there must have been
light to enter it. The immense vegetable deposits in the Carboniferous
period announce the introduction of an extensive terrestrial vegetation;
and the impressions left by the wood and leaves of the trees show
that these first forests must have grown in a damp soil and a moist
atmosphere. In short, all the remains of animals and plants hidden in
the rocks have something to tell of the climatic conditions and the
general circumstances under which they lived, and the study of fossils
is to the naturalist a thermometer by which he reads the variations of
temperature in past times, a plummet by which he sounds the depths of
the ancient oceans,--a register, in fact, of all the important physical
changes the earth has undergone.

But although the animals of the early geological deposits indicate
shallow seas by their similarity to our shoal-water animals, it must not
be supposed that they are by any means the same. On the contrary, the
old shells, crustacea, corals, etc., represent types which have existed
in all times with the same essential structural elements, but under
different specific forms in the several geological periods. And here
it may not be amiss to say something of what are called by naturalists
_representative types_.

The statement that different sets of animals and plants have
characterized the successive epochs is often understood as indicating
a difference of another kind than that which distinguishes animals now
living in different parts of the world. This is a mistake. There are
so-called representative types all over the globe, united to each other
by structural relations and separated by specific differe