1898] NOTES AND COMMENTS 153 
trated by nineteen admirable plates. Professor Hill’s work now 
conclusively demonstrates that there has been no connection across 
the Isthmus of Panama since the Oligocene period, and that there 
has not been any very extensive submergence in post-Jurassic times. 
The connection between the Carribbean and the Pacific must be 
restricted to a very limited connection in the Eocene and Oligocene 
epochs. All the interesting theories which explain English glacia- 
tion by a Pleistocene submergence of the Panama, and consequent 
diversion of the Gulf Stream, may therefore be finally dismissed as 
apocryphal. 
MIGRATION AND HOoMOTAXIS © 
GEOLOGISTS know well enough that identity of species in widely 
separated fossil faunas does not imply identity of age, since time 
must be allowed for the species to have migrated from one locality 
to the other. It therefore becomes important to know the actual 
rate of migration of marine species at the present day, Opportuni- 
ties of observation are not often presented. One occurred when the 
Suez Canal put the waters of the Mediterranean into communication 
with the Red Sea. Another began on Feb, 3, 1825, when the 
Limfjord Denmark, up till then a fresh-water lake, became con- 
nected with the North Sea and its fauna began to change. A 
study of the immigrant animals was published by J. Collin at Copen- 
hagen in 1884, and a notice of more recent changes has recently 
been contributed by Th. Mortensen to Videnskabelige Meddelelser 
(1897, pp. 311-319). Since 1884 the following species are known 
to have entered the Limfjord: Raia batis, Actaeon tornatilis, 
Clavellina lepadiformis, Portunus arcuatus, Cribrella sanguinolenta, 
Ophioglypha albida, and Echinocardium cordatum. But even yet the 
fauna is not assimilated to that of the adjoining sea; many echino- 
derms have yet to find their way in, such as the sand-stars 
Amphiura filiformis, Ophiopholis aculeata, and Ophiothrizx fragilis, 
We may expect too that some seventy species of molluscs will enter 
before long, for long ago when the Limfjord was an arm of the sea, 
and not yet a fresh-water lake, many of these species lived in it and 
their shells are found along its margin. In the history of the earth 
a century is but “a watch in the night”; nevertheless the time 
needed for a species to pass as it were into the next street may 
suggest to the geologist how long a similar form must have taken 
to traverse the width of an ocean. 
STUDIES IN AUXOLOGY 
THE growth of marine animals is a study attended with difficulty, 
since thorough observation of individuals is only possible in an 
aquarium, and there the conditions are inevitably unnatural. Dr 
C. G. J. Petersen, the director of the Danish Biological Station, has 
