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horses, Eohippus and Orohippus (Fig. 22, F F'), then existed, and that they had four usable

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toes, with the rudiment of a fifth (in the case of Eohippus), on the fore-foot, and three on the hind-foot.

Fig. 23. - Fore and Hind Feet of the Modern Horse and of Fossil Ilorses.
A, Modern Horse.

F', Orohippus

did, second and fourth finger B, Piohippus.

a, canon-bone ) of third

and toe, c. Protohippus and flipparion. 6, pastern

e, little finger.

finger and D. Miolippiis and Anchitleriun. coronary boue

toe E Mesolippus.

coffiu-bone

In the number of toes, then (and in other respects besides), these first-known representatives of the horse family resembled ordinary quadrupeds far more closely than does the horse of to-day.

Then, in the middle part of the age, we find that there were horses (Mesohippus) with one rudimentary and three useful toes on the forefoot, the middle toe being much the largest. In the latter part of the age, there flourished still another genus of horse (Hipparion), with three toes on the fore-foot, but only the middle toe useful; the two outside of it, like the dewclaws in cattle, not touching the ground. Still another genus (Pliohippus) has the rudimentary toes intermediate in development between those of Hipparion and those of our domestic horse.

The latter is also represented, among the fossils of the region where the last two genera above mentioned have been found, by a genus nearly or quite the same as the modern horse.

There is to be found in all, according to Professor 0. C. Marsh (the highest authority on this subject), among the fossils of Wyoming, the Upper Missouri region, the Rocky-Mountain region, and Oregon, a series of not less than thir

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ty intermediate or transitional forms. These 1 make thoroughly clear the process of modified descent by which the advance was made from the five-toed Eohippus to the horse of to-day.

Nor is the marked progress throughout the series confined to the solidification of the foot, and consequent increase in capacity for speed. It is also noticeable in the gain 'in size,2 the lengthening of the neck, and the modification of the skull.

That such a closely linked set of forms, illustrating the progressive development of an animal of as high grade and greatly specialized powers as the horse, from so comparatively inferior an animal as the little Eohippus, should ever have been discovered, is all that the most sanguine believer in the development theory could have desired, and more than he could fairly have expected. One more example may be introduced in this place, to show how species and genera are formed by descent with modification, under the influence of a varying environment.

Among the many forms of shell-fish (mollusks) that flourished in the seas of preceding geological ages there have been none so remarkable, at once for their size and for their exquisitely ornamented shells, as the ammonites. Roughly resembling the pearly nautilus of the seas of to-day, the ammonites far excelled the latter both in size and in beauty, since their diameter at times was no less than four feet, and the surface of the shell was ornamente with delicate sinuous tracings. One family of these ammonites, the Arietidæ, has been studied by not less than four generations of palæontologists, whose observations and conclusions agree well enough to form the basis of a lemarkably detailed and convincing argument for organic evolution just set forth by Professor Hyatt. It is utterly impossible to reproduce in an elementary book like the present one, even a summary of this masterly discussion. No one but an expert zoologist and palæontologist could follow the structural details of this richly illustrated monograph, and nothing less than a reproduction of its whole series of lithographic cuts could convey to the reader any idea of the wealth of material upon which the argument is based. It must here suffice to

1 Examples hardly less striking are to be found among the fossil horses of Europe.

2 Eohippus was about the size of a fox.

1 Among whom are Von Buch, Quenstedt, D'Orbigny, Fraas, Oppel, Hyatt, Neumayr, and Wähner.

2 Genesis of the Arietidæ, by Alpheus Hyatt, Washington, Smithsonian Institute, 1889.

state a few of the conclusions drawn from thollsands of carefully studied specimens of the fossil shell-fish, of which only here and there a form is reproduced in the accompanying plate (see frontispiece), for the use of which the author's are indebted to the kindness of Professor Hyatt. In the first place, it must be understood that, beginning at the earliest times in the Silurian age, the seas, down to the present day, have never failed to exhibit some kind of large shellfish of flattened spiral form, sometimes strongly disk-shaped, and with a general likeness in structure and mode of growth to the nautilus already referred to. These great shell-fish reached their highest point, as regards number of individuals and species, size, complexity of structure and of embryological development toward the middle of the Mesozoic era, afterward declining to their position of comparative unimportance in the marine fauna of today.

Although the consistent evolutionist would naturally expect, as the evidence comes to be more and more completely collected and classified, to find an increasing amount of proof of the community of descent of all these coiled and discoidal mollusks, from the earliest Silu

1 Such straight forms as the Orthoceras would, of course, belong in the same series.

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