Croll estimates that
about sixty million years have elapsed since the Cambrian period, but this,
judging from the small amount of organic change since the commencement of
the Glacial epoch, appears a very short time for the many and great
mutations of life, which have certainly occurred since the Cambrian
formation; and the previous one hundred and forty million years can hardly
be considered as sufficient for the development of the varied forms of life
which already existed during the Cambrian period. It is, however,
probable, as Sir William Thompson insists, that the world at a very early
period was subjected to more rapid and violent changes in its physical
conditions than those now occurring; and such changes would have tended to
induce changes at a corresponding rate in the organisms which then existed.
To the question why we do not find rich fossiliferous deposits belonging to
these assumed earliest periods prior to the Cambrian system, I can give no
satisfactory answer. Several eminent geologists, with Sir R. Murchison at
their head, were until recently convinced that we beheld in the organic
remains of the lowest Silurian stratum the first dawn of life. Other
highly competent judges, as Lyell and E. Forbes, have disputed this
conclusion. We should not forget that only a small portion of the world is
known with accuracy. Not very long ago M. Barrande added another and lower
stage, abounding with new and peculiar species, beneath the then known
Silurian system; and now, still lower down in the Lower Cambrian formation,
Mr Hicks has found South Wales beds rich in trilobites, and containing
various molluscs and annelids.
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