TIME AND CHANGE 
tom, now undreamed of, and in volcanic eruptions 
as great as any in the past. Sucha 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 Paleozoic and Mes- 
ozoic 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 ap- 
proximately 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 
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