modern naturalist had no small assurance, who
declared "that individuals alone were capable of destruction, and that
species were so perpetuated that nature could not annihilate them, so
long as the planet lasted, or at least that nothing less than the shock
of a comet, or some similar disaster, could put an end to their
existence."[949] The Italian geologist, on the contrary, had satisfied
himself that many species of Testacea, which formerly inhabited the
Mediterranean, had become extinct, although a great number of others,
which had been the contemporaries of those lost races, still survived.
He came to the opinion that about half the species which peopled the
waters when the Subapennine strata were deposited had gone out of
existence; and in this inference he does not appear to have been far
wrong.

But, instead of seeking a solution of this problem; like some other
geologists of his time, in a violent and general catastrophe, Brocchi
endeavoured to imagine some regular and constant law by which species
might be made to disappear from the earth gradually and in succession.
The death, he suggested, of a species might depend, like that of
individuals, on certain peculiarities of constitution conferred upon
them at their birth; and as the longevity of the one depends on a
certain force of vitality, which, after a period, grows weaker and
weaker, so the duration of the other may be governed by the quantity of
prolific power bestowed upon the species which, after a season, may
decline in energy, so that the fecundity and multiplication of
individuals may be gradually lessened from century to century, "until
that fatal term arrives when the embryo, incapable of extending and
developing itself, abandons, almost at the instant of its formation, the
slender principle of life by which it was scarcely animated,--and so all
dies with it."

Now we may coincide in opinion with the Italian naturalist, as to the
gradual extinction of species one after another, by the operation of
regular and constant causes, without admitting an inherent principle of
deterioration in their physiological attributes. We might concede, "that
many species are on the decline, and that the day is not far distant
when they will cease to exist;" yet deem it consistent with what we know
of the nature of organic beings, to believe that the last individuals of
each species retain their prolific powers in their full intensity.

Brocchi has himself speculat