CUMULATIVE EVIDENCE FOR EVOLUTION
gressively and continuously. All the evidence in our possession indicates that, apart from occasional relativly sudden, more or less accidental short cuts, the course of solar evolution is almost inconceivably slow.
Passing from suns to lesser material units, let us consider the evolution of planets. The lifetime of a planet as compared with that of a sun is short, and its evolution is correspondingly 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 continents, changes due to earthquakes, floods, and the vicissitudes 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 historical 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 the 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 periodically 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 lessened weight of the continents brings about another squeez-
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