lion 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. The presence
of phosphatic nodules and bituminous matter, even in some of the
lowest azotic rocks, probably indicates life at these periods; and
the existence of the Eozoon in the Laurentian formation of Canada is
generally admitted. There are three great series of strata beneath the
Silurian system in Canada, in the lowest of which the Eozoon is found.
Sir W. Logan states that their "united thickness may possibly far
surpass that of all the succeeding rocks, from the base of the
palaeozoic series to the present time. We are thus carried back to a
period so remote, that the appearance of the so-called primordial
fauna (of Barrande) may by some be considered as a comparatively modern
event." The Eozoon belongs to the most lowly organised of all classes of
animals, but is highly organised for its class; it existed in countless
numbers,