486 
S1LURIA. 
[Chap. XX. 
Another hypothesis which has been advanced in opposition to the mass 
of positive evidence is, that, although such earlier rocks are void of Ich- 
thyolites, the sediments may all have been formed in limited zones around 
the earth ; just as it is believed by some naturalists that there are seas 
subject to certain currents and conditions of the bottom in which no Pishes 
are now living. Here, again, the application of such a theory is still 
more negatived by the facts adduced. Silurian rocks similar in structure, 
and containing the same organic remains, are not confined to any one seg- 
ment of the earth's surface, however broad, but are largely developed in 
nearly all known regions. The argument is therefore untenable in face 
of the knowledge we have acquired, that, amidst a profusion of all the 
other forms of marine life, Pishes and other Yertebrates are absent from 
strata formed in the seas of this early age. 
The prevalence of a widely spread primeval ocean, and the existence 
of a land-surface not yet subjected to such variations of outline as have 
since brought about and modified the different climates of the earth, 
when connected with a belief in the former greater radiation of heat 
from its interior, are the chief data required to satisfy us that in the Silu- 
rian period physical conditions prevailed with which the nature and exten- 
sive spread of the earlier groups of animals are in harmony. 
Admitting that in the remote periods during which the earliest acces- 
sible sediments were accumulated, large areas of laud (though probably 
of comparatively little altitude), as well as vast rivers, must have existed 
as sources of the primeval deposits, we may still well believe that such 
lands were separated by wide seas, and that hence we ought necessarily to 
meet with a smaller number of littoral animals and a greater number of 
oceanic forms. Taking advantage of the great prevalence of the sup- 
posed pelagic forms, some persons have suggested that we may have as yet 
discovered only the deep-sea products of the Silurian period, and that, 
when the true edges of its lands come to be detected, we may then find 
Plants and many creatures now unknown to us. To this I reply that many 
proofs have already been adduced of lands which were contiguous to the 
marine Silurian sediments, both Lower and Upper, and even to the earlier 
Cambrian. Innumerable pebble-beds, coral-reefs, and trails of animals 
that crawled upon the tide-marked mud, are the principal evidences re- 
quired ; and to these I will again presently advert *. 
It has also been said that the great number of floating shells, particu- 
larly the Orthocerata, which abounded in the Silurian era are in them- 
selves indicators of deep seas, remote from land, into which, therefore, 
terrestrial spoils were little likely to be transported. Now the Silurian 
Chambered Shells may, indeed, have required a certain depth of water ; and 
yet many of them, like the Nautilus pompilius of the present day, might 
* These proofs are ho numerous that in a recent essay Agassiz treats of all the fossils of the period 
as having been accumulated on what he calls 'The Silurian Beach!' 
