Boox V.]_ DOCTRINE OF COLONIES. 629 
The occurrence of two very distinct faunas in two closely consecutive 
series of strata does not prove that the one abruptly died out and the 
other suddenly appeared in its place. It only shows, as Darwin has 
so well enforced, the imperfection of the geological record. In the 
interval between the formation of two such contrasted groups of 
rocks the fauna of the lower strata must have continued to exist 
elsewhere, and gradually to change into the newer facies which 
appeared when sedimentation recommenced with the upper strata. 
Distinct zoological provinces have no doubt been separated by 
narrow barriers in former geological periods, as they still are to-day. 
There seems, therefore, every probability that such migrations 
as M. Barrande has supposed in the case of the Silurian fauna of 
Bohemia have again and again taken place. 
That examples of these migrations have not been more frequently 
observed arises doubtless from the inherent imperfection of the geological 
record and from the difficulty of obtaining the requisite paleontological 
and stratigraphical data. But that remarkable instances of precursory 
appearances, apparently complete disappearances, and long subsequent 
reappearances of fossil forms have been chronicled among the stratified 
formations can admit of no doubt. One of the most interesting of these 
may be quoted here from its bearing on the Bohemian evidence of 
M. Barrande. Among the Lower Silurian rocks of the south of Scotland 
eertain black anthracitic shales have long been known to extend for 
many miles along the strike of the strata from Moffatdale towards the 
north-east and south-west. They contain a profusion of graptolites, 
which, however, are almost wholly confined to these dark bands. The 
associated grey shales, greywackes, and grits are usually barren of 
organic remains, but on every horizon of black shales the graptolites 
reappear. The total maximum thickness of the black-shale group may 
be from 400 to 500 feet. Over these strata comes a series of massive 
ereywacke, grit, and blue and grey shale, with a thickness of at least 
8000 or 10,000 feet, in which hardly any trace of an organism has 
been met with, though in some of the gritty and calcareous bands 
encrinites, petraia, trilobites, and a few brachiopods have been obtained. 
Next in succession lies another zone of black shale, in which the same 
staptolites once more reappear in extraordinary abundance. These 
organisms could evidently only flourish in the black carbonaceous mud. 
When the conditions for the deposit of this sediment ceased the grapto- 
lites died out in the district, though they continued to live in other 
areas where they could find their appropriate habitat. No sooner, 
however, did the dark mud spread once more over the district than the 
sraptolites swarmed in again and re-occupied their former sites. The 
interval of time represented by the 8000 or 10,000 feet of strata between 
the two black-shale zones must have been great, if estimated in years, 
“yet it seems to have been accompanied with but little change in the 
graptolite fanna, though a few species occur in the later which have 
not been met with in the older zone.! 
1 The order of succession of these Silurian strata has been worked out in detail by 
the officers of the Geological Survey across the whole of the south of Scotland, and has 
been established by an overwhelming mass of evidence. 
