The longest line of human ancestry must hide its diminished head before the pedigree of this insignificant shell-fish.  We Englishmen are proud to have an ancestor who was present at the Battle of Hastings.  The ancestors of Terebratulina caput serpentis may have been present at a battle of Ichthyosauria in that part of the sea which, when the chalk was forming, flowed over the site of Hastings.  While all around has changed, this Terebratulina has peacefully propagated its species from generation to generation, and stands to this day, as a living testimony to the continuity of the present with the past history of the globe.

Up to this moment I have stated, so far as I know, nothing but well-authenticated facts, and the immediate conclusions which they force upon the mind.  But the mind is so constituted that it does not willingly rest in facts and immediate causes, but seeks always after a knowledge of the remoter links in the chain of causation.

Taking the many changes of any given spot of the earth’s surface, from sea to land and from land to sea, as an established fact, we cannot refrain from asking ourselves how these changes have occurred.  And when we have explained them—­as they must be explained—­by the alternate slow movements of elevation and depression which have affected the crust of the earth, we go still further back, and ask, Why these movements?

I am not certain that any one can give you a satisfactory answer to that question.  Assuredly I cannot.  All that can be said, for certain, is, that such movements are part of the ordinary course of nature, inasmuch as they are going on at the present time.  Direct proof may be given, that some parts of the land of the northern hemisphere are at this moment insensibly rising and others insensibly sinking; and there is indirect, but perfectly satisfactory, proof, that an enormous area now covered by the Pacific has been deepened thousands of feet, since the present inhabitants of that sea came into existence.  Thus there is not a shadow of a reason for believing that the physical changes of the globe, in past times, have been effected by other than natural causes.  Is there any more reason for believing that the concomitant modifications in the forms of the living inhabitants of the globe have been brought about in other ways?

Before attempting to answer this question, let us try to form a distinct mental picture of what has happened in some special case.  The crocodiles are animals which, as a group, have a very vast antiquity.  They abounded ages before the chalk was deposited; they throng the rivers in warm climates, at the present day.  There is a difference in the form of the joints of the back-bone, and in some minor particulars, between the crocodiles of the present epoch and those which lived before the chalk; but, in the cretaceous epoch, as I have already mentioned, the crocodiles had assumed the modern type of structure.  Notwithstanding