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 Homotaxis 



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 Videnskabcligc Mcddclclscr 

 (1897, pp. 311-319). Since 1884 the following species are known 

 to have entered the Limfjord : Baia batis, Actaeon tornatilis, 

 ClaveUina lepadiformis, Portunus arcuatus, CribreUa so mgv. inolenta, 

 Ophioglypha albida, and JEchinocardium cordcttam. 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 Jiliformis, Ophiopholis acidccda, and Ophiothrlc 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 



