ss ee 2 “ x  — 
3 Ms 4 
~Boow V.| ~ DOCTRINE OF COLONIES. 627 
Cretaceous flora of Aix la Chapelle, with its numerous dicotyledong, 
has a much more modern aspect than the contemporaneous fauna. 
In the Western Territories of North America much controversy has 
been raised as to the position of the “Lignitic series,” its rich 
terrestrial flora having an undoubted Tertiary facies, while its fauna 
is Cretaceous. According to Fuchs the most important turning- 
point in the history of the plant-world is to be found not, as in the 
case of the terrestrial fauna, between the Sarmatian stage and the 
Congeria-beds, but on an older horizon, namely between the first and 
second Mediterranean stage.’ 
From what has now been stated it will be understood that the 
existence of any living species or genus of plant or animal within a 
certain geographical area is a fact which cannot be explained except 
by reference to the geolugical history of that species or genus. The 
existing forms of life are the outcome of the evolution which has 
been in progress during the whole of geological time. From this 
‘point of view the investigations of paleontological geology are 
invested with the profoundest interest, for they bring before us the 
history of that living creation of which we form a part. 
vi. Doctrine of Colonies.—M. Barrande, the distinguished 
author of the Systeme Salurien de la Boheme, drew attention more 
than a quarter of a century ago to certain remarkable intercala- 
tions of fossils in the series of Silurian strata of Bohemia. He 
showed that, while these strata presented a normal succession of 
organic remains, there were nevertheless exceptional bands, which, 
containing the fossils of a higher zone, were yet included on different 
horizons among inferior portions of the series. He termed these 
precursory bands “ colonies,” and defined the phenomena as consisting 
in the partial co-existence of two general faunas, which, considered 
as a whole, were nevertheless successive. He supposed that during 
the later stages of his second Silurian fauna in Bohemia the first 
phases of the third fauna had already appeared, and attained some 
degree of development in some neighbouring but yet unknown 
region. At intervals, corresponding doubtless to geographical 
changes, such as movements of subsidence or elevation, volcanic 
eruptions, &c., communication was opened between that outer region 
and the basin of Bohemia. During these intervals a greater or less 
number of immigrants succeeded in making their way into the 
Bohemian area, but as the conditions for their prolonged continuance 
there were not yet favourable, they soon died out, and the normal 
fauna of the region resumed its occupancy. The deposits formed 
during these partial interruptions, notably graptolitic schists, and 
calcareous bands, accompanied by igneous sheets, contain, besides 
the invading species, remains of some of the indigenous forms. 
Eventually, however, on the final extinction of the second fauna, 
and, we may suppose, on the ultimate demolition of the physical 
1 KH. Weiss, Newes Jahrb. 1878, p. 180; also Z. Deutsch. Geol. Ges, xxix. p. 252, 
28 2 
