400 ELEMENTARY GEOLOGY 
ing line between certain periods, as the Devonian and 
Silurian, is not at all easily drawn, although at some 
distance above or below, there is no difficulty in telling 
by the fossils contained in the rocks, which is Devo- 
nian and which Silurian. 
When we speak of age in the geological time-scale, 
we do not mean a period of exact time, as in the 
ordinary use of the term. We cannot measure the 
history of the earth im years; but the time-scale serves 
as a convenient means of dividing this history into 
stages. Similarly the early history of man is divided 
into Paleolithic and Neolithic stages, based upon the 
different kinds of instruments he used as he was 
developing from savagery to civilization. 
Nor do we mean, when speaking of the Silurian of Europe, 
America, and Australia, that in these three widely separated 
continents, the Silurian represents the same age in realtime. At 
present the animals of Australia differ greatly from those of 
other continents ; and probably in the past, as now, very different 
kinds of animals existed simultaneously in widely separated 
areas. Still the rocks of a given age in different regions were 
undoubtedly formed at nearly the same time. 
What similarity of fossils really does show, is the stage in 
development which the animals of that period had reached. At 
some time animal life, the earth over, had attained a certain 
stage of development. In different quarters of the globe this 
might have been at very different times, for animals may have 
developed more rapidly in Europe, for example, than in Australia. 
Again, we may borrow an analogy from the history of man. 
Even a half-century ago, men in various parts of the earth were 
