THE CHILDHOOD OF THE EARTH 



earth runs through a swarm of them on its pathway 

 round the Sun. But they are very small. Some of 

 them are no bigger than a slate pencil. Few are as 

 big as brickbats, and nearly all are burnt up by 

 the air-friction before they reach the earth's surface. 

 Larger ones still fall on the earth, however, and it is 

 calculated that many hundreds reach us in fragments 

 every year. But when the earth was young many 

 thousands fell every day. 



To this era, or immediately before it, belongs the birth 

 of the Moon. It is a subject of interest to geologists, 

 because it is admitted that the materials of which the 

 Moon is constituted are similar to those of the earth ; 

 and it is believed that its history up to a certain point 

 was very like that of the earth. It had its great vol- 

 canic era such as we have described; but its develop- 

 ment closed shortly afterwards. We are considering, 

 however, at this moment its origin. It was once part 

 of the earth. All of us have read of those little 

 animals, of which one form is the amoeba and another 

 form the white corpuscles of the blood. If we watch 

 them under the microscope we may see one of them 

 slowly lengthen out, then break in two, and each part 

 go swimming away by itself, a perfect animal. It was 

 Sir G. H. Darwin, F.K.S., who proved mathematically in 

 1879 that the origin of the Moon was such that we 

 may properly compare it to the splitting up of the 

 little animals just described. The date of this event 

 cannot be fixed even approximately beyond saying 

 that as astronomical events go it must have been 



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