492 LE, JOURNAL OF GEOLOGY: 
connected with many of the now scattered islands of the Indian 
Archipelago, but was separated from Asia. 
Deep seas afford no barriers to rapid swimmers or to any 
pelagic forms, orto dwellers in the deeps. These approach much 
more nearly universal distribution, the first two being checked 
only by continental barriers, and the latter only by shallow 
waters. 
CRITERIA OF MIGRATIONS. 
Occurrence of identical or very closely related species and 
faunas in widely separated localities is good evidence of migra- 
tion from one of these localities to the other, or from another 
region to both of them. In most cases, however, what are called 
identical species are in reality only representative forms. 
As long as ideas about stratigraphic units were loose, and 
species had too broad a significance, it was impossible to recog- 
nize slight changes in faunas and age. Oppel* from his study of 
the Jura gave to geology the idea of stratigraphic zones, each 
with its characteristic animals. He recognized the importance 
of selecting types of great horizontal and narrow vertical distri- 
bution, and for this reason, where it was possible, chose ammon- 
ites as zone fossils. 
This work would never have been possible had not Barrande? 
introduced an extremely narrow idea of species. _Neumayr car- 
ried this study further by introducing the idea of ‘‘mutations”’ 
for genetic series of forms occurring in succeeding zones. 
The use of these principles enables the naturalist to draw 
sharp lines between successive faunas. 
Sporadic occurrence.—It is often noticed that species or faunas 
are intermittent in their occurrence, and this too when it is not 
due to difference of facies. Just such a case is the reappearance 
of the Waverly fauna in higher beds of the Lower Carboniferous 
of Missouri and Arkansas, already alluded to in this paper. The 
intercalations of marine beds in the fresh-water Che DOBLIS of the 
Coal Measures is another good example. 
t Juraformation. 
2Systéme Silur. Centre Bohéme. 
