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628 PALAONTOLOGICAL GEOLOGY.  [Boox V. _ 
barriers hitherto only occasionally and temporarily broken, the third — 
fauna, which had already sent successive colonies into the Bohemian — 
area, now swarmed into it, and peopled it till the close of the Silurian 
period.* 
‘This original and ingenious doctrine has met with much opposi- — 
tion on the part of geologists and paleeontologists. Of the facts cited — 
by M. Barrande there has been no question, but other explanations 
have been suggested for them. It has been said, for example, that 
the so-called colonies are merely bands of the Upper Silurian rocks 
or third fauna, which by great plications or fractures have been so 
folded with the older rocks as to seem regularly interstratified with 
them,” the fossils of the colonies showing little or no mixture of 
Lower Silurian fossils, such as might have been expected had 
they been really coeval. But the author of the Systeme Silurien 
contends that of such foldings or fractures there is no evidence, but — 
that, on the contrary, the sequence of the strata appears normal and 
undisturbed. Again, it has been urged that the difference of organic 
contents in these so-called colonies is due merely to a difference in 
the conditions of water and sea-bottom, particular species appearing 
with the conditions favourable to their spread, and disappearing 
when these ceased. But this contention is really included in M. 
Barrande’s theory. ‘The species which disappear and reappear in 
later stages must have existed in the meanwhile outside of the area 
of deposit, which is precisely what he has sought to establish. It 
has been further alleged that no other examples have ever been 
found of the fauna of one distinct geological formation appearing 
unmixed in a formation of older date. Much of the opposition 
which his views have encountered has probably arisen from the feeling 
that if they are admitted they must weaken the value of paleeonto- 
logical evidence in defining geological horizons. <A paleontologist, 
who has been accustomed to deal with certain fossils as unfailing 
indications of particular portions of the geological series, is naturally 
unwilling to see his generalizations upset by an attempt to show 
that the fossils may occur on a far earlier horizon. | 
If, however, without entering into the details of the Bohemian 
instances, we view this question from the broad natural history 
platform from which it was regarded by M. Barrande, it is impossible 
not to admit that such phenomena as he has sought to establish in 
Bohemia must have often occurred in all geological periods and in 
all parts of the world. No one now believes in the sudden extinction 
and creation of entire faunas. Every great fauna in the earth’s 
history must have gradually grown out of some pre-existing one, 
and must have insensibly graduated into that which succeeded. 
! The doctrine of colonies is developed in the “Systéme Silurien du centre de la 
Bohtme,” 1852, i. p. 78; ‘* Colonies dans le bassin Silurien de la Bohéme,” in Bull. Soe. 
Géol. France (2nd ser,), xvii. (1859), p. 602; “ Défense des Colonies,” Prague, i. (1861), 
ji. (1862), iii. (1865), iv. (1870), v. (1881). 
* This contention has recently been revived by Mr. J. E. Marr, who hag gone over 
the ground in dispute in Bohemia. Q. J. Geol. Soc. Nov. 1880, p. 605. 
