and plants whose existence
and maintenance required such a state of things, more than by any
positive knowledge respecting it. Where we find 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 mistak