IIO EVOLUTION OF LIFE. 
Primary, Secondary, and Tertiary Ages, with their charac- 
teristic fossils, according to many Geologists, were not 
confined to America, but extended all over the globe; the 
whole earth having passed at the same time successively 
through the Primary, Secondary, and Tertiary Ages. Some 
few Geologists do not accept this onion-coat hypothesis, 
which supposes that similar rocks, with similar remains, 
were deposited at the same time all round the earth, like 
the layers in the coat of an onion. With all deference to 
Geologists, let us examine the tests used for determining 
the time of the deposition of foreign rocks as compared 
with our own. The test of having similar minerals, when 
applied to elucidating the age of foreign rocks as compared 
with those on this continent, is worthless, since the chalk, 
sandstone, etc. of which the rocks are composed are form- 
ing in all ages; while a determination of the age of rocks, 
based on the order in which they follow or overlie one 
another,—when applied, for example, to New York and 
England, separated by an ocean,—to say the least, is very 
unreliable. The third test, that rocks having similar 
organic remains are of the same age, considered by most 
Geologists as settling the question, whenever such com- 
parisons are possible, may be as fruitful a source of error 
as the view that similar minerals deposited in the same way 
are of the same age. Nor does the reverse of this propo- 
sition hold good, that rocks are of a different age because 
they contain different fossils. Suppose, for example, that the 
western part of North America and Australia were gradually 
to sink into the sea, as parts of the world are now doing, 
and then slowly to rise again, the Geologist of an indefinitely 
remote future might argue, because he found many fossil 
pouch-bearing animals in Australia, and the bones of an 
extinct human race in America, that the Kangaroo was not 
contemporaneous with the Indian. From the distribution 
of plants and animals at the present time, we know that 
