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EXPLANATIONS. 



cultivators of science should allow themselves to follow 

 the dictates of reason, against the behests of prejudices 

 unworthy of them and of their age. 



Time is the true key to difficulties regarding appear- 

 ances of determinateness in species. Few of us, not even 

 geologists, have ever realized in our minds the extent of 

 time which has elapsed since the beginning of life upon 

 this globe. Mr. Lyell, without intending to favor the de- 

 velopment theory, lends us powerful testimony on this 

 point. After showing reason to believe that about thirty- 

 five thousand years have passed since the Niagara began 

 to cut down the rock through which it flows, during which 

 time the living mollusks, whether marine or terrestrial, 

 are proved to have undergone no change, he thus proceeds 

 — " If such events can take place, while the zoology of 

 the earth remains almost stationary and unaltered, what 

 ages may not be comprehended in those successive tertia- 

 ry periods during which the Flora and Fauna of the globe 

 have been almost entirely changed ! Yet how subordi- 

 nate a place in the long calendar of geological chronology 

 do the successive tertiary periods themselves occupy ! 

 How much more enormous a duration must we assign to 

 many antecedent revolutions of the earth and its inhabi- 

 tants ! No analogy can be found in the natural world to 

 the immense scale of these divisions of past time, unless 

 we contemplate the celestial spaces which have been 

 measured by the astronomer. Some of the nearest of these 

 within the limits of the solar system, as, for example, the 

 orbits of the planets, are reckoned by hundreds of mil- 

 lions of miles, which the imagination in vain endeavors to 

 grasp. Yet one of these spaces, such as the diameter of 

 the earth's orbit, is regarded as a mere unit, a mere infin- 

 itesimal fraction of the distance which separates our sun 

 from the nearest star. By pursuing still further the same 

 investigations, we learn that there are luminous clouds, 

 scarcely distinguishable by the naked eye, but resolvable 

 by the telescope into clusters of stars, which are so much 

 more remote, that the interval between our sun and Sirius 

 may be but a fraction of this larger distance. To regions 

 of space of this higher order in point of magnitude, we 

 may, probably, compare such an interval of time as that 

 which divides the human epoch from the origin of the co- 

 ralline limestone, over which the Niagara is precipitated 



