organic remains were fragmentary relics. Our museums exhibit fossil
shells of immeasurable antiquity, as perfect as the day they were
formed, whole skeletons without a limb disturbed--nay, the changed
flesh, the developing embryos, and even the very footsteps of primieval
organisms. Thus the naturalist finds in the bowels of the earth
species as well defined as, and in some groups of animals more numerous
than, those that breathe the upper air. But, singularly enough, the
majority of these entombed species are wholly distinct from those that
now live. Nor is this unlikeness without its rule and order. As a
broad fact, the further we go back in time the less the buried species
are like existing forms; and the further apart the sets of extinct
creatures are the less they are like one another. In other words,
there has been a regular succession of living beings, each younger set
being in a very broad and general sense somewhat more like those which
now live.

It was once supposed that this succession had been the result of vast
successive catastrophes, destructions, and re-creations 'en masse'; but
catastrophes are now almost eliminated from geological, or at least
palaeontological speculation; and it is admitted on all hands that the
seeming breaks in the chain of being are not absolute, but only relative
to our imperfect knowledge; that species have replaced species, not in
assemblages, but one by one; and that, if it were possible to have all
the phenomena of the past presented to us, the convenient epochs and
formations of the geologist, though having a certain distinctness, would
fade into one another with limits as undefinable as those of the
distinct and yet separable colours of the solar spectrum.

Such is a brief summary of the main truths which have been established
concerning species. Are these truths ultimate and irresolvable facts,