PREFACE. vii 



" Eurycolpic folds," a term recently suggested to me, and which 

 I have intentionally introduced into the present work in preference 

 to either " graben," or " rift valleys," both of which are obviously 

 misnomers, since the valleys in question are generally produced 

 by folding, due to lateral pressure, and not either through rifting, 

 or vertical collapse. 



Since our return, and in connection with both the first and second 

 Tanganyika expeditions, the large number of new fishes obtained 

 have been elaborately worked out by Mr. Boulenger of the British 

 Museum. The descriptions contained in the present volume 

 being his simply, while the more conspicuous genera and species 

 have been accurately illustrated by Mr. Green. The Crustacea 

 were examined by Messrs. Cunnington and Caiman, the Sponges 

 by Mr. Evans, Professor Minchin and Herr Weltner in Berlin. 

 For the description of the Mollusca, the Polyzoa and Protozoa, 

 I myself have been responsible, but I have received unlimited 

 help from Professor Ray Lankester himself, Mr. Edgar Smith 

 and others at the British Museum. The anatomy of the cele- 

 brated jelly-fish was examined some years ago by Mr. Giinther 

 in Oxford, and in what appears in the present volume, I have 

 consequently been able to draw largely upon his original descrip- 

 tion, only adding some drawings of the living Medusa together 

 with information about its life history and development, which I 

 acquired on the spot. We have been further indebted to Mr. 

 Prior for the complete identification of the rock specimens collected 

 by Mr. Fergusson ; and, as I have explained in the text, we have 

 been most materially helped by Mr. Huddleston in the comparison 

 of the Tanganyika gastropods with the shell remains of the Jurassic 

 seas. Last, but not least, I have to thank Professor G. B. Howes, 

 who after my return from both my first and second Tanganyika 

 expeditions, allowed me the unlimited use of the Huxley laboratory 

 in the Royal College of Science and of his own experience and 

 advice. 



The title which I have chosen for the present work, " The Tan- 

 ganyika Problem," expresses the fact that there is a puzzle, a 

 mystery, attached to Tanganyika, and the elucidation of this 

 mystery has formed the single motive for several independent 

 lines of enquiry described in the present volume. The Tan- 



