HYDROGR. SECTION — APPENDIX D ( 60 ) 
place in the same order. This periodicity is in very many cases caused by 
fact that the species in question, though always living in the said areas, yet 
reach an abundant development under such circumstances as are especially favo 
able to them. In other cases the periodicity is caused by the circumstan > that 
the species are brought to the said areas by currents. Some of the la > 
organisms have been used to throw light upon the origin as well as 
locity of the currents. | 
Amsterdam, March, 1906 
The Hydrographical Section 
a 
