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of descent were true; while such facts as the free-swimming condition of the young oyster, and the possession of the occasionally injurious rudiment of a tail, and of the worm-shaped appendage in man, are inexplicable by any other theory. We must, in short, recognize the important truth discovered by Agassiz and widely applied, under the name of the law of biogenesis, by Professor Haeckel, that the development of every individual is a brief repetition of the development of its tribe.

CHAPTER VII.

THE TESTIMONY OF GEOLOGY.

EFORE the evidence of geology for or

against evolution can be summed up, it may be well to outline a few of the facts of paleontology, or the ancient life-history of the earth.

It is admitted by geologists that our globe was in a melted conditicn for a time after it assumed its present shape of a flattened sphere. Of course no life of any kind conceivable by us could have existed on its surface till after the latter had become solid, and then had cooled to a temperature below that of boiling water. How long a time has elapsed since this hardening of the earth's crust took place is a matter still under discussion : 1 indeed, the estimates vary all the way from two hundred millions of years to ten millions. Much confidence has been placed in Sir William Thomson's limit of one hundred millions of years as the greatest length of time that can be allowed; and both the method which he employed, and his wellknown ability to deal with problems of this character, l'ender Thomson's estimate at least as authoritative as any that has yet been made. It is quite impossible to realize what is meant by a hundred million years, or what geological changes might be produced in such a period of time. Let us see what night be done in one direction alone by geological forces acting throughout such a period. Coast-lines have been known to rise at the rate of nearly four inches a year: suppose such an elevation to take place for a hundred million years, at the very gradual rate of a tenth of an inch per year, and we shall have an upheaval of almost one hundred and sixty miles above sea-level! ERAS.

1 A summary of recent opinions on this subject may be found in Geikie's Text-Book of Geology, pp. 51-56.

Whatever may be the length of time during which the condition of the earth's surface has been such as to allow the existence of life upon it, geologists, excepting those who would reckon the ages of the cenozoic together, calling the whole but one, are agreed upon the following divisions into eras of life, and ages under them :

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AGES.

1. Quaternary, or age of man.

CENOZOIC, or era of newer forms of life.

2. Tertiary, or age of mammals 1 and of

angiosperms. 1

MESOZOIC, or era
of intermediate Age of reptiles and of cycads.5
forms of life.

1. Carboniferous, or age of amphibians 1

and of acrogens. 1 PALÆOZOIC, era of ancient 2. Deronian, or age of fishes. forms of life.

or

3. Silurian, or age of mollusks1 and of

algæ.1
4. Cambrian, or age of trilobites.?

Eozoic.3

Eozoic, or age of beginnings of life.

The time-ratios of the palæozoic, mesozoic, and cenozoic eras, are, according to Professor Dana,+ 12: 3: 1; that is, if the total time from the beginning of the silurian to the present were estimated at forty-eight million years, the palæozic would occupy thirty-six million, the mesozoic nine million, and the cenozoic three million. Or, to put it in the form of a diagrain :

I D'fiure in Appendix.

2 Tuobites were crustacean animals not ulike gigantic sow-bugs in their general appearance.

3 The life-lustory of the Eozoic is not, and may lierer be, clearly made out.

• Manual of Geology, p. 591. 5 Defined pp. 111, 112.

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Of the supposed life of the eozoic nothing is known.

The silurian age began with no animals higher in the scale than shell-fish.

The seas swarmed with these and many other low forms of life; but backboned animals do not seem to have existed, and no plants higher than seaweeds remain to us in the rocks deposited at that early time. Land-plants first occur in rocks formed near the close of the silurian age,

1 It is always to be understood that when the statement is made, that any form of life began at a certain time, it means that the earliest known fossils occur in rocks of that age.

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