4 EVOLUTION OF THE EARTH 



those of the present. On the whole, the problem of the genesis 

 of the earth appears to He somewhat more in the field of the 

 geologist than in that of the astronomer. 



The Place of the Earth in the Universe 

 The earthy a member of the solar system. The earth is 

 but one among the planets which together with the sun con- 

 stitute the solar system. It is neither the largest nor the 

 smallest, neither nearest to nor most remote from the sun. 



The sun is a star and is but one among the millions of stars, 

 and, though apparently so great as seen from the earth, is 

 mediocre among them in size and brightness. The origin of 

 the earth is obviously bound up with the origin of the other 

 planets and all in the history of the sun. A presentation of 

 the significant facts of magnitudes, motions, and distribution 

 of these bodies, familiar though they are to most readers, 

 should therefore precede the consideration of the genesis of 

 the earth. 



The planets visible to the unassisted eye are, besides the 

 earth, ^Yt in number, distinguished by the ancients from the 

 stars by their steady light and by their wanderings through 

 the zodiacal path in the sky, wanderings produced as a result 

 of the combined effect of their motions and that of the earth in 

 nearly circular paths about the sun, their common center. The 

 telescope has added to the number of planets two large ones, 

 Uranus and Neptune, invisible to the naked eye because of 

 their distance from sun and earth, and, in one zone of inter- 

 mediate distance, a swarm of smaller bodies, the asteroids, 

 better called planetoids. In size the planets sink to vanishing 

 insignificance in comparison with the sun or any other star. 

 Their distances from the sun and from each other are also 

 almost infinitesimal in comparison with the distances which 

 separate the stars. They shine by light reflected from the 



