least the provisional assent of all the best thinkers of the day--the
hypothesis that the forms or species of living beings, as we know them,
have been produced by the gradual modification of pre-existing
species--then the existence of persistent types seems to teach us
much. Just as a small portion of a great curve appears straight, the
apparent absence of change in direction of the line being the exponent
of the vast extent of the whole, in proportion to the part we see; so,
if it be true that all living species are the result of the modification
of other and simpler forms, the existence of these little altered
persistent types, ranging through all geological time, must indicate
that they are but the final terms of an enormous series of
modifications, which had their being in the great lapse of pregeologic
time, and are now perhaps for ever lost.

In other words, when rightly studied, the teachings of palaeontology are
at one with those of physical geology. Our farthest explorations carry
us back but a little way above the mouth of the great river of Life:
where it arose, and by what channels the noble tide has reached the
point when it first breaks upon our view, is hidden from us.

The foregoing pages contain the substance of a lecture delivered before
the Royal Institution of Great Britain many months ago, and of course
long before the appearance of the remarkable work on the "Origin of
Species" just published by Mr. Darwin, who arrives at very similar
conclusions. Although, in one sense, I might fairly say that my own
views have been arrived at independently, I do not know that I can
claim any equitable right to property in them; for it has long been my
privilege to enjoy Mr. Darwin's friendship, and to profit by
corresponding with him, and by, to some extent, becoming acquainted with
the workings of his singularly original and well-stored mind. It was