STUDIES FOR STUDENTS. 493 
In the Jura of northern Europe, according to Neumayr,* the 
genera Lytoceras and FPhylloceras appear only sporadically, being 
lacking entirely in sixteen zones; and the known species in that 
region do not belong to a genetic series. But in southern Europe 
these genera appear plentifully in all the zones, and do repre- 
sent genetic series. 
The migration northward at several successive periods is thus 
clearly established. In these same beds the genus Amaltheus 
also appears intermittently; but no region is yet known where 
Amaltheus developed continuously. Among the Jurassic ammon- 
ites of northern Europe there are a number of cryptogenic types, 
most of which coincide with the Amaltheide in their appearance 
and therefore came from the same region. 
These facts go to show that the principles on which Bar- 
rande’s? doctrine of colonies was founded were not far wrong, 
although it has since been found that his colonies were only 
younger rocks carried into the midst of older beds by disloca- 
tions.3 Barrande’s idea was that younger faunas are sometimes 
intercalated with older ones by changes in physical geography. 
‘The modern doctrine of colonies on the other hand is that older 
faunas have often been preserved by survival in places where no 
great changes have taken place, and that these older faunas have 
been intercalated with the younger by changes in physical 
geography. 
Hleterochronous appearance.—By heterochronous appearance of 
forms is meant their occurrence at different horizons in separated 
regions. Now it is well known that a species seldom occurs at 
exactly the same time in two widely separated places; it must 
originate at the one and migrate towards the other. This migra- 
tion takes time, and the species may be entirely extinct at the 
point of origin before it reaches the second place. But geolo- 
gists have commonly used species as criteria of horizons, when in 
‘Jahrb. K. K. Geol. Reichsanstalt Wien. Bd. 28, 1878. Ueber unvermittelt 
auftretende Cephalopodentypen im Jura Mittel-Europas. 
? Systéme Silur. Centre Bohéme, Vol. I., p. 73, etc. 
3J. E. Marr, Quart. Jour: Geol. Soc., 1880, p. 605, 1882, p. 313. 
