important truth so well insisted upon by Lyell and by Darwin--the
imperfection of the geological record. It can be demonstrated that the
geological record must be incomplete, that it can only preserve remains
found in certain favourable localities and under particular conditions;
that it must be destroyed by processes of denudation, and obliterated by
processes of metamorphosis. Beds of rock of any thickness, crammed full
of organic remains, may yet, either by the percolation of water through
them, or by the influence of subterranean heat, lose all trace of these
remains, and present the appearance of beds of rock formed under
conditions in which living forms were absent. Such metamorphic rocks
occur in formations of all ages; and, in various cases, there are very
good grounds for the belief that they have contained organic remains,
and that those remains have been absolutely obliterated.

I insist upon the defects of the geological record the more because
those who have not attended to these matters are apt to say, "It is all
very well, but, when you get into a difficulty with your theory of
evolution, you appeal to the incompleteness and the imperfection of the
geological record;" and I want to make it perfectly clear to you that
this imperfection is a great fact, which must be taken into account in
all our speculations, or we shall constantly be going wrong.

You see the singular series of footmarks, drawn of its natural size in
the large diagram hanging up here (Fig. 2), which I owe to the kindness
of my friend Professor Marsh, with whom I had the opportunity recently
of visiting the precise locality in Massachusetts in which these tracks
occur. I am, therefore, able to give you my own testimony, if needed,
that the diagram accurately represents what we saw. The valley of the
Connecticut is classical ground for the geologist. It contains great
beds of sandstone, covering many square miles, which have evidently