CUMULATIVE EVIDENCE FOR EVOLUTION 



gressively and continuously. All the evidence in our posses- 

 sion indicates that, apart from occasional relatively sudden, 

 more or less accidental short cuts, the course of solar evolu- 

 tion is almost inconceivably slow. 



Passing from suns to lesser material units, let us consider 

 the evolution of planets. The lifetime of a planet as com- 

 pared with that of a sun is short, and its evolution is cor- 

 respondingly much more rapid, so rapid that many of the 

 events of its career can be noted by man. In our own planet, 

 for example, we have observed changes in the levels of con- 

 tinents, changes due to earthquakes, floods, and the viscissi- 

 tudes of climate. Similar but far more extensive changes are 

 recorded accurately in the strata of the earth's crust. 



An intelligent perusal of the rocky pages of our earth's his- 

 torical record shows that the hills are not eternal but are 

 periodically coming into being and passing away; that the 

 oceans change their depths and contours; that the continents 

 join hands for a time and then part company; that parts of. 

 continents become islands and that islands become attached 

 to continents. If viewed by a being whose time passes as 

 slowly as it passes for the sun, the earth would appear to be 

 in a continual state of flux. It would seem to pulsate like a 

 gigantic heart as the continental and oceanic areas periodi- 

 cally expand and contract. During the periods of relatively 

 expanded continents and contracted seas the lands in many 

 regions are high and mountainous. These high lands and 

 mountains undergo a long, slow period of gradation, during 

 which much of their solid material is washed into the sea. 

 The filling up of the sea helps to make it overflow upon the 

 lowest parts of the continents, and thus the area of the oceans 

 increases and that of the continents decreases. Before this 

 goes very far the increased weight of the sea floor and the les- 

 sened weight of the continents brings about another squeez- 



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