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Updated: May 1, 2025
They are then barely distinguishable from the Condylarthra and Creodonta, and seem only recently to have issued from a common ancestor with those groups. In the course of the Tertiary we find them especially in South America, which was cut off from the North and its invading Carnivores during the Eocene and Miocene developed into large sloths, armadilloes, and anteaters.
The events of the days of creation are recorded here, but they are days of such length that they are to be reckoned only in millions of years. The evolution of the horse, according to the best and latest research, from the eohippus of Eocene times a small mammal no larger than the fox to the proud and fleet creature that we prize to-day, occupied four or five millions of years.
The later slates, too, are degenerates, and much of the sandstones have the hearts of prodigals. In the Bad Lands of Arizona I could have cut my way into some of the Eocene formations with my pocket-knife. Apparently the farther away we get from the parent granite, the more easily is the rock eroded.
Not long ago, palaeontologists maintained that the whole class of birds came suddenly into existence during the eocene period; but now we know, on the authority of Professor Owen, that a bird certainly lived during the deposition of the upper greensand; and still more recently, that strange bird, the Archeopteryx, with a long lizard-like tail, bearing a pair of feathers on each joint, and with its wings furnished with two free claws, has been discovered in the oolitic slates of Solenhofen.
We are led, therefore, to inquire whether there was a time in the history of the Mediterranean, since the Eocene period, when the waters were lower than at present. That this was the case we have clear evidence.
In the beginning of this century it was one of the canons of the popular geological creed that the first warm-blooded quadrupeds which had inhabited this planet were those derived from the Eocene gypsum of Montmartre in the suburbs of Paris, almost all of which Cuvier had shown to belong to extinct genera.
It appears that of more than fifty species of these foraminifera described by D'Archiac, one or two species only are found in other tertiary formations whether of older or newer date. Nummulites intermedia, a Middle Eocene form, ascends into the Lower Miocene, but it seems doubtful whether any species descends to the level of the London clay, still less to the Argile plastique or Woolwich beds.
"Moreover, the lines of both groups to a certain extent approximate, but, within the limits of our knowledge, they do not meet. . . Was the order according to which the introduction of new forms seems to have taken place since the Eocene then entirely changed, or did it continue as far back as the period when these lines would have been gradually fused in a common centre?"
A quarter of a million years has perhaps elapsed since the beginning of that cold period which, at all events in this part of the earth, killed Palaeolithic man; yet how small a part of his racial life even that time would seem if, as some believe, his remains may be traced as far back as the Eocene!
Hence some modifications have been required in the classifications founded on less perfect materials. The Eocene, Miocene, and Pliocene periods have been made to comprehend certain sets of strata of which the fossils do not always conform strictly in the proportion of recent to extinct species with the definitions first given by me, or which are implied in the etymology of those terms.
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