But the admission of the
doctrine of colonies implies the further admission that even identity
of organic remains is no proof of the synchronism of the deposits
which contain them.
4. The discussions touching the _Eozoon_, which commenced in 1864,
have abundantly justified the fourth proposition. In 1862, the oldest
record of life was in the Cambrian rocks; but if the _Eozoon_ be,
as Principal Dawson and Dr. Carpenter have shown so much reason for
believing, the remains of a living being, the discovery of its true
nature carried life back to a period which, as Sir William Logan has
observed, is as remote from that during which the Cambrian rocks were
deposited, as the Cambrian epoch itself is from the tertiaries. In
other words, the ascertained duration of life upon the globe was
nearly doubled at a stroke.
5. The significance of persistent types, and of the small amount of
change which has taken place even in those forms which can be shown to
have been modified, becomes greater and greater in my eyes, the longer
I occupy myself with the biology of the past.
Consider how long a time has elapsed since the Miocene epoch. Yet, at
that time, there is reason to believe that every important group in
every order of the _Mammalia_ was represented.
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