time during which the different
species were gradually demarcated from one another.

Some of the ammonites, which belonged to this cuttle-fish group, soon
attained a very considerable size; but a shell known as the orthoceras
(I wish my subject didn't compel me to use such _very_ long words, but I
am not personally answerable, thank heaven, for the vagaries of modern
scientific nomenclature) grew to a bigger size than that of any other
fossil mollusk, sometimes measuring as much as six feet in total length.
At what date the gigantic cuttles of the present day first began to make
their appearance it would be hard to say, for their shell-less bodies
are so soft that they could leave hardly anything behind in a fossil
state; but the largest known cuttle, measured by Mr. Gabriel, of
Newfoundland, was eighty feet in length, including the long arms.

These cuttles are the only invertebrates at all in the running so far as
colossal size is concerned, and it will be observed that here the
largest modern specimen immeasurably beats the largest fossil form of
the same type. I do not say that there were not fossil forms quite as
big as the gigantic calamaries of our own time--on the contrary, I
believe there were; but if we go by the record alone we must confess
that, in the matter of invertebrates at least, the balance of size is
all in favour of our own period.

The vertebrates first make their appearance, in the shape of fishes,
towards the close of the Silurian period, the second of the great
geological epochs. The earliest fish appear to have been small,
elongated, eel-like creatures, closely resembling the lampreys in
structure; but they rapidly developed in size and variety, and soon
became the ruling race in the waters of the ocean, where they maintained
their supremacy till the rise of the great secondary saurians. Even
then, in spite of the severe competition thus introduced, and still
later, in spite of the struggle for life against the huge modern
cetaceans (the true monarchs of the recent seas), the sharks continued
to hold their own as producers of gigantic forms; and at the present day
their largest types probably rank second only to the whales in the whole
range of animated nature. There seems no reason to doubt that modern
fish, as a whole, quite equal in size the piscine fauna of any previous
geological age.

It is somewhat different with the next great vertebrate group, the
amphibians, represented in our own