PREFACE. 



The numerous and successful efforts made in recent times to 

 extend, and as far as possible to complete, our knowledge of the 

 globe we inhabit, constitute one of the most characteristic 

 features of the present age. The central parts of great con- 

 tinents, hitherto untrodden by the foot of civilised man, are only 

 now commencing to be systematically explored, and, while the 

 interest of the general reader is stimulated from time to time 

 by the description of newly-discovered regions, a rich harvest 

 of fresh materials is placed at the disposal of the scientific 

 student. 



The work carried on with so much energy and success on 

 terra firma has been supplemented in the domain of the sea by 

 several naval expeditions, sent out for the especial purpose of 

 fathoming the depths of the ocean, of observing the currents 

 and the physical and chemical condition of the water, of 

 bringing up from the bottom samples of the deposits now in 

 process of formation, and of gathering specimens of the countless 

 forms of animal life with which the sea abounds at all depths. 



As a natural consequence of the new facts brought to light 

 day after day, opinions held until recently by the most com- 

 petent authorities in almost every branch of Natural Science, 

 but especially in Biology, Geology, and Physical Geography, 

 have undergone considerable modifications, or have had to be 

 abandoned altogether. On the other hand, the numerous 

 carefully-made observations collected in every quarter of the 

 globe furnish an opportunity for attempting, with renewed 



