CONTENTS. 



CHAPTER I. 



INTRODUCTORY. 



PAGE 

 Reasons for acquiring a better knowledge of the fauna of the Great African Lakes — 

 Our first knowledge of the existence of unique animals in Tanganyika arose as a 

 by product of Burton's journey — The shells collected by Speke on Tanganyika 

 were marine in aspect — But the shells obtained from the other African Lakes 

 were not so — Extension of our knowledge of the Tanganyika shells — Possibility 

 of the lake having been connected with the sea — Intensification of the interest 

 in the Tanganyika problem produced by Boehm's discovery of jelly-fishes — 

 Formation of the first Tanganyika expedition — As a result of this it is shown 

 that the Tanganyika problem is larger than was supposed — Conflict between 

 the zoological evidence and geological anticipations — Erroneous character of 

 Murchisoa's geological speculations — Actual impressions of the African in- 

 terior — Remarkable character of the Halolimnic shells of Tanganyika — Their 

 similarity to those of the Jurassic seas — This comparison a possible solution of 

 the Tanganyika problem — Insufficiency of our knowledge of the Great African 

 Lakes — Suess' views — Possibility of a former extension of the sea into Tan- 

 ganyika from the north — Geological and geographical interest of the country 

 north of Tanganyika — Formation of the second Tanganyika expedition — 

 Arrangement of matters contained in the present work ..... I 



CHAPTER II. 



ON THE NATURE AND ORIGIN OF FRESH-WATER FAUNAS AND THEIR 

 RELATION TO THE FAUNA OF THE SEA. 



Animals live in fresh-water which do not inhabit the sea — Animals live also in 

 brackish water — It is generally accepted that fresh-water organisms have 

 arisen from marine forms and have colonized fresh-waters from the ocean 

 — Recent additions to our knowledge of fresh-water faunas — Beaudants' experi- 

 ments — Views of Semper Sollas and Von Martens — Apparent impossibility of 

 many marine organisms ever getting into fresh-water — Inadequacy of existing 

 views respecting the nature of fresh-water faunas — Similarity of fresh-water 

 faunas all over the world — -Distinction of primary and secondary fresh-water 

 stocks — Origin of the second obvious, of the first not so clear — Ancestral 



