crust appear to be separated by millions of years.  Away back in pre-Cambrian times, there appear to have been immense periods during which the peace and repose of the globe were as profound as in our own time.  Then at the end of Palaeozoic time—­how many millions of years is only conjectural—­the truce of aeons was broken, and the dogs of war let loose; it was a period of revolution which resulted in the making of one of our greatest mountain-systems, the Appalachian, and in an unprecedented extinction of species.  Later eras have witnessed similar revolutions.  Why may they not come again?  The shrinking of the cooling globe must still go on, and this shrinking must give rise to surface disturbances and dislocations, maybe in the uplift of new mountain-ranges from the sea-bottom, now undreamed of, and in volcanic eruptions as great as any in the past.  Such a shrinkage and eruption made the Hawaiian Islands, probably in Tertiary times; such a shrinkage may make other islands and other continents before another period of equal time has elapsed.

Of course the periods and eras into which the geologists divide geologic time are as arbitrary as the months and seasons into which we divide our year, and they fade out into each other in much the same way; but they are really as marked as our seasonal divisions.  Not in their climates—­for the climate of the globe seems to have been uniformly warm from pole to pole, without climatic zones, throughout the vast stretch of Palaeozoic and Mesozoic times—­but in the succession of animal and vegetable life which they show.  The rocks are the cemeteries of the different forms of life that have appeared upon the globe, and here the geologist reads their succession in time, and assigns them to his geologic horizons accordingly.  The same or allied forms appeared upon all parts of the earth at approximately the same time, so that he can trace his different formations around the world by the fossils they hold.  Each period had its dominant forms.  The Silurian was the great age of trilobites; the Devonian, the age of fishes; Mesozoic times swarm with the gigantic reptiles; and in Tertiary times the mammals are dominant.  Each period and era has its root in that which preceded it.  There were rude, half-defined fishes in the Silurian, and probably the beginning of amphibians in the Devonian, and some small mammalian forms in the Mesozoic time, and doubtless rude studies of the genus Homo in Tertiary times.  Nature works up her higher forms Jike a human inventor from rude beginnings.  Her first models barely suggest her later achievements.

In the vegetable world it has been the same; from the first simple algae in the Cambrian seas up to the forests of our own times, the gradation is easily traced.  Step by step has vegetable life mounted.  The great majority of the plants and animals of one period fail to pass over into the next, just as our spring flowers fail to pass over into summer, and our summer flowers into fall.  But the law of evolution is at work, and life always rises on stepping-stones of its dead self to higher things.