480 THE POPULAR SCIENCE MONTHLY. 



pose, be more than a few thousand years old. Five thousand years 

 nearly exhausts all historical time. Ten thousand years certainly does. 

 Though we have no earlier historical record, yet other records are not 

 wanting. Geology tells us that ten thousand years is but a mere mo- 

 ment in the span of the earth's history. We learn from geology that 

 even the career of man himself has lasted far more than ten thousand 

 years. Yet man is but the latest addition to the succession of life on 

 the earth. For the chronology of the earlier epochs of the earth's his- 

 tory we require majestic units to give adequate expression to our dates. 

 Thousands of years are not sufficient, nor tens of thousands, nor hun- 

 dreds of thousands. The course of geological time is to be reckoned 

 in millions of years. 



The corridors of time through which I wish to give you a glimpse 

 are these dignified millions. Yet our retrospect will only extend to a 

 certain definite epoch in the past history of our earth. We speak of 

 nothing anterior to the time when our earth assumed the dignity of 

 maternity, and brought forth its first and only child. We shall trace 

 the development of that child which, though millions of years old, is 

 still in dependence on its parent. We shall describe the influence of 

 the parent over the child, and the not less remarkable reaction of the 

 child upon the parent. We shall foreshadow the destiny which still 

 awaits the mother and child when millions of years shall have elapsed. 



At the time of its birth the earth was not, as we see it now, clothed 

 with vegetation and teeming with animal life. It was a huge inor- 

 ganic mass, too hot for life, perhaps hot enough to be soft or viscid, if 

 not actually molten. The offspring was what might be expected from 

 such a parent. It was also a rude inorganic mass. Time has Avrought 

 wondrous changes in both parent and child. Time has transformed 

 the earth into an abode of organic life. It has transformed the earth's 

 offspring into our silvery moon. 



It will be my duty to sketch for you the manner in which these 

 changes have been brought about. To a great extent we can do this 

 with no hesitating steps ; we are guided by a light which can not de- 

 ceive. It is the light of mathematical reasoning. These discoveries 

 are of an astronomical character, but they have not been made by tele- 

 scopes. They have been made by diligent labors of the most abstruse 

 kind. The mathematical astronomer sits at his desk, and not in an 

 observatory. He has in his hand a pen, and not a telescope. Before 

 him lies a sheet of paper, and not the starry heavens. He is no doubt 

 furnished with a few facts from observation. It is his province to 

 interpret those facts, to inform them with life, and to infer the un- 

 known from the known. It is thus discoveries are made which are the 

 Bublimesl efforts of human genius. 



The argument on which I invite you to follow me is founded on a 

 very simple matter. Many of those present go every summer to the 

 sea-side. Those who do so are well acquainted with the daily ebb and 



