whole series the organic life of the earth has undergone a
corresponding alteration. This alteration also has been gradual, but
complete; after a certain interval not a single species existing which
had lived at the commencement of the period. This complete renewal of
the forms of life also appears to have occurred several times:--That
from the last of the geological epochs to the present or historical
epoch, the change of organic life has been gradual: the first appearance
of animals now existing can in many cases be traced, their numbers
gradually increasing in the more recent formations, while other species
continually die out and disappear, so that the present condition of the
organic world is clearly derived by a natural process of gradual
extinction and creation of species from that of the latest geological
periods. We may therefore safely infer a like gradation and natural
sequence from one geological epoch to another.

Now, taking this as a fair statement of the results of geological
inquiry, we see that the present geographical distribution of life upon
the earth must be the result of all the previous changes, both of the
surface of the earth itself and of its inhabitants. Many causes, no
doubt, have operated of which we must ever remain in ignorance, and we
may, therefore, expect to find many details very difficult of
explanation, and in attempting to give one, must allow ourselves to call
into our service geological changes which it is highly probable may have
occurred, though we have no direct evidence of their individual
operation.

The great increase of our knowledge within the last twenty years, both
of the present and past history of the organic world, has accumulated a
body of facts which should afford a sufficient foundation for a
comprehensive law embracing and explaining them all, and giving a
direction to new researches. It is about ten years since the idea of
such a law suggested itself to the writer of this essay, and he has
since taken every opportunity of testing it by all the newly-ascertained
facts with which he has become acquainted, or has been able to observe
himself. These have all served to convince him of the correctness of his
hypothesis. Fully to enter into such a subject would occupy much space,
and it is only in consequence of some views having been lately
promulgated, he believes, in a wrong direction, that he now ventures to
present his ideas to the public, with only such obvious