August 24, 1905J 



NA TURE 



415 



The forms of the first and second categories may be 

 entirely neglected in dealing with the distribution of fresh- 

 water fishes. Their range is regulated by the sea, and 

 they must be dealt with in conjunction with littoral forms. 

 Eighty-si,x species in the list of African fresh-water fishes 

 belong to these categories. 



The third category is of secondary interest in the history 

 of the fresh-water fauna ; but, as in the case of Galaxias, 

 forms referred to it may give rise to discussion. 



It is with the members of the fourth category that we 

 shall mainly deal in the portion of this Address which is 

 devoted to the origin and mode of dispersal of the African 

 fishes. 



The Polvpterid.e. — This is incontestably the most re- 

 markable family of African fishes. Entirely restricted to 

 Tropical Africa and the Nile, without any known near 

 allies, living or e.\tinct, its history is one of the greatest 

 riddles in ichthyology. From the evolutionary point of 

 view, no group is of greater interest, owing to its prob- 

 able relation to the Chondropterygians or Elasmobranchs, 

 to the Osteolepid Crossopterygians, out of which the Lung- 

 fishes seem to have been evolved, and to the earliest 

 pentadactyle vertebrates, the Stegocephalous Batrachians. 

 •Although generally brigaded by modern systematists with 

 the Osleolepida in the order Crossopterygii, it is still 

 doubtful whether it should not rank as a distinct order, 

 Cladistia of Cope, the characters which differentiate it 

 from these early Teleostomes being perhaps of greater 

 importance than those which separate these from the 

 Dipneusti. Until we have some proof to the contrary, we 

 are justified in regarding the Polypteridse as having arisen 

 in Africa from fresh-water ancestors, themselves derived 

 from early Mesozoic types which are entirely hypothetical. 



The Lepidosirenid.^. — Protopterus in Africa and Lepido- 

 siren in South America are specialised modifications of the 

 Ceratodontidas, still represented by one species in Australia, 

 which have left remains in Triassic, RhEetic, Jurassic, and 

 Cretaceous rocks of Europe, North America, Patagonia, 

 North and South Africa, India, and Australia. The dis- 

 tribution of the Ceratodontidje has therefore been, at 

 different periods at least, a world-wide one, and we should 

 feel justified in assuming the living representatives of the 

 Lepidosirenida; to have been evolved out of this family 

 independently in Africa and in South America. On the 

 other hand, in view of the old age of the group, there is 

 no reason why the LepidosirenidEe should not have passed 

 from one of the present continents into the other when 

 they were connected by land. As Protopterus is a less 

 specialised type than Lepidosiren, the probabilities would 

 then be that the former originated in Africa. Mr. 

 Lydekker, in his " Geographical History of Mammals," 

 states his opinion that Lepidosiren reached its present 

 habitat by way of Africa. The mode of life of these fishes 

 renders them less dependent on hydrographical systems, 

 and the distribution of the species, which cannot yet be 

 traced in a satisfactory manner, is evidently very different 

 from that of other groups. 



The Mormyrid.'E. — This extraordinary group, of which 

 so many new and remarkable types have been discovered 

 within the last few years, especially in the Congo, is 

 peculiar to the fresh waters of Tropical Africa and the 

 Nile. Its morphology shows it to be highly specialised 

 from some very lowly Teleostean ancestor. This I 

 believed to be found in the Albulida;, a family already re- 

 presented in Cretaceous seas, and of which one species 

 still occurs on the West Coast of Africa. But Dr. Ride- 

 wood, who has recently made a much more careful study 

 of the cranial characters of the two families, is unable 

 to support the suggestion of a direct descent from the 

 .Albulidoe. It nevertheless remains probable that the 

 Mormyridas were derived from forms more closely allied 

 to the known Albulidse than to any other family with which 

 we are acquainted, and which no doubt lived in Cretaceous 

 seas : and we may therefore assume that the Mormyridse 

 originated in Africa, and were evolved out of Cretaceous 

 marine ancestors. 



The Notopterid^. — This is another eccentric family, 

 having many points in common with the MormyridEE and 

 with the North American Hyodontidie. It is represented 

 by five species, three of which live in the Indo-Malay 



region and two in Tropical Africa. Its derivation is still 

 a mystery. The fact that its most specialised form 

 (Xenomystus) is African, and that a species differing but 

 little from the living Notopterus occurs in fresh-water 

 deposits in Sumatra, which are regarded by some geologists 

 as of Middle Eocene age — although, as stated further on 

 d propos of the Cyprinidee, there is reason for regarding 

 them as Miocene, or even later — justifies us in believing, 

 until further pateontological evidence be available, that the 

 African forms are immigrants from the East. 



The Osteoglossid.ie. — An archaic type of Teleosteans, 

 now represented by two genera in South America, by one 

 in Australia and the Malay Archipelago, and by a fourth 

 in Tropical Africa and the Nile. Excellently preserved 

 fossils from the Middle Eocene of Wyoming (Dapedo- 

 glossus) are most nearly allied to, but more generalised 

 than, the Australian-Malay genus ; whilst the less satis- 

 factorily known British Lower Eocene Brychajtus appears 

 nearer to the South American Arapaima. The African 

 genus Heterotis is the most specialised form. The Osteo- 

 glossidK are evidently an ancient group, now in process 

 of extinction, which once had a very wide distribution. 

 The fact of the only known fossil representatives being 

 from North America and Europe does not seem sufficient 

 evidence of the northern origin of the family, as suggested 

 by Mr. Lydekker. 



. P.-iNTODONTiD^E, Phractol.jemid;e, CromeriiDjE. — Three 

 monotypic families peculiar to Africa. The first bears a 

 near relationship to the Osteoglossidte, and was probably 

 derived from them ; but the two others, discovered within 

 the last few years, are so aberrant and isolated among 

 the Malacopteryglans that we are absolutely in the dark 

 as to their possible origin. 



The Characinid.^. — This is one of the larger groups of 

 African fishes — with ninety-three species, referred to twenty 

 genera, mostly from the Nile and Tropical Africa, as far 

 east as the great lakes, but only very sparsely represented 

 in East and South Africa. 



One of the most striking features of the South American 

 fresh-water fish-fauna is the extraordinary number and 

 variety of forms of the Characinids, unquestionably one of 

 the most lowly and generalised groups of exclusively fresh- 

 water Teleosts. There occur in that part of the world 

 as many as 500 species (about two-fifths of the whole 

 fresh-water fish-fauna), divided among some sixty genera. 

 The carnivorous forms predominate, but the herbivorous 

 or semi-herbivorous are also very numerous. The latter 

 would evidently compete with the Cyprinids, their near but 

 more specialised relatives, which are so numerously re- 

 presented in North America ; and it is a remarkable fact 

 that not a single Cyprinid is known to extend further 

 south than Guatemala. 



Although palaeontology has taught us nothing respecting 

 the Characinids, we have reason to assume, from the 

 morphological point of view, that they were the precursors 

 of the Cyprinids, which, we know, were already abundantly 

 represented in North America and Europe in Lower 

 Tertiary times, when the Isthmus of Panama was under 

 the sea. When, in the Miocene, North and South America 

 became re-united, the waters of the latter part of the world 

 must have been already so fully stocked with Characinids 

 as to prevent the southern spread of the Cyprinids. This 

 is the only explanation that can be offered of the total 

 absence of Cyprinids in South America, considerations of 

 climate being of no avail in view of their distribution all 

 over Africa. If, therefore, the Characinids existed in pro- 

 fusion in South America before the Miocene period, we 

 are justified in claiming for them a high antiquity, and 

 by putting it at the Upper Cretaceous we need not fear 

 going too far back. 



The CYPRINID..E. — These fishes, as mentioned above, are 

 very closely related to the preceding, and there is every 

 reason to believe the former to be derived from the latter. 

 Their least specialised genera (Catostominje) are now 

 found in North and Central America (about sixty species), 

 whilst three species, referable to the same genera, inhabit 

 Eastern Siberia and China. These Catostominae are known 

 to have had representatives in the Eocene of North 

 America, whilst the more specialised CypriniuEe, which 



NO. 1869, VOL. 72] 



