380 
SILUEIA. 
[Chap. XV. 
the Lower Silurian fauna prevailed in Bohemia, were carried to these spots by 
currents, and thus for a time inhabited the region before the more general in- 
troduction of exotic species at a subsequent period (see also p. 375). 
Some geologists would endeavour to explain the fact by showing that fossil 
animals are frequently associated with particular physical conditions, disap- 
pearing and reappearing with changes of the sea-bottom. The difference be- 
tween this view and that of M. Barrande is, that he considers his ancient ' Colony ' 
to have migrated from remote seas, where his Third or Upper-Silurian Fauna 
already nourished*. 
Having now fully reconsidered all the evidence brought forward by M. Barrande 
in his well-reasoned memoirs, I am of opinion that he has fairly substantiated 
the soundness of his view of the former migration of species from seas more or 
less distant — in other words, that animals having deserted certain waters, and 
migrated to other localities, returned long after and under favouring conditions 
to the region they had long before inhabited. 
In his general comparison of the Silurian fauna of Bohemia with that of Bri- 
tain, M. Barrande shows, as before said, the impossibility of identifying each of the 
bands of his rich basin with the subdivisions established in the original Silurian 
region. He justly recognizes only the great divisions of Lower and Upper Silu- 
rian, placing in the earliest of these his First and Second Faunas, and his Third 
Fauna in the latter. With this general agreement (the application may be made 
to very distant tracts containing rocks of this age), it now appears certain, from 
M. Barrande's clear explanation of the relations of the Bohemian strata and 
their respective contents, that his Second Fauna, the representative of the Llan- 
deilo and Caradoc of Britain, without any mixture of other remains, is super- 
posed to a band containing animals which belong to his Third Fauna, or Upper 
Silurian. This fact sets aside the inference that his ' Colonies ' may represent the 
Llandovery rocks of British classification, and shows that in Bohemia it is im- 
practicable to establish a ' Middle Silurian,' as suggested by Mr. Salter when I 
was preparing the last edition of ' Siluria.' Irrespective of the ingenious esta- 
blishment of the natural-history fact of the migration of animals, it is clear that 
these alternations of life indisputably connect the Lower and Upper Silurian 
rocks in one system, through an interchange of a considerable number of their 
respective fossils. M. Barrande justifies me, indeed, in not employing the term 
( Middle Silurian ; ' for although I have fully recognized the Llandovery rocks 
as occupying a transitional place in my own country, I well know that there 
are many tracts in which no such evidences exist ; and the reader who refers to 
the table on the side of the coloured map will at once perceive that the Upper 
Llandovery is bracketed as the base of the Upper Silurian, and the Lower Llan- 
dovery as the summit of the Lower Silurian. 
The fossil species collected by M. Barrande in his rich Bohemian basin are 
thus enumerated : — Fishes, 5 (1 in Stage f, and 4 in Stage g, both equivalents of 
Ludlow rocks) ; Trilobites, 350 ; Crustacea, 70 Cephalopods, 870 ; Pteropods, 
70 ; Gasteropods, 550 ; Bivalves, 450 ; Brachiopods, 220 ; Polyzoa and Polyps, 
150. Total, about 2735 species. 
Before I take leave of this rich centre of Silurian life, which my eminent 
friend has rendered so classical, let me invite the attention of the reader to one 
salient proof of his acumen and sagacity as a naturalist. Every one knows 
that living Crustaceans, from the King-crab (Limulus) and Lobster down- 
wards, proceed from eggs ; and before reaching maturity, many of them, even of 
* See ' Bulletin Soc. Ge"ol. de France,' vol. xvii. p. 639, and vol. xx. p. 492 ; also Barrande, ' Defense 
dea Colonies.' 
