4i8 



NA TURE 



[August 24, 1905 



undescribed, has been re-discovered by Budgett, and is now 

 being studied. Should it prove to be related to the Tan- 

 ganyika species, it would also have to be regarded as a 

 relic of the same Eocene sea, and it would add further 

 support to the very simple explanation which 1 have 

 ventured to offer of a case which seemed so tremendously 

 puzzling in our previous state of ignorance of the geo- 

 logical conditions of Africa between the equator and the 

 tropic of cancer. 



.As explained by Prof. Cornet, Tanganyika has been 

 until verv recent times without an outlet. The Lukuga, 

 which drains into the Congo, was only formed after Lake 

 Kivu became, owing to volcanic eruption, a tributary of 

 the Tanganyika through the Rusisi River. The greater 

 or less salinity of the water of a lake without an outlet is 

 a matter of course, and therefore Tanganyika was for a 

 long time a salt lake. Its water is still, Mr. Moore says, 

 somewhat salt. No wonder that the Cichlids, which else- 

 where in .Africa show no aversion to such conditions, and 

 which somehow or other contrive to settle into isolated 

 waters, should have been among the first inhabitants of 

 the lake, where, without having to face competition with 

 other types of fishes, they throve and became differentiated 

 into a multitude of genera. When the hydrographical 

 conditions changed and the water gradually lost its salinity, 

 first on the surface and later at greater depths, an influx 

 of other forms of fish-life (Polypterus, Characinids, 

 Cvprinids, Silurids, &-c.) penetrated into the lake, some 

 from the Nile system through the Rusisi, others from the 

 Congo up the Lukuga. This explains w-ell enough the 

 character of the Tanganyika fish-fauna. The Cichlids, 

 the oldest inhabitants of the lake, nearly all belong to 

 endemic species, many of which constitute genera repre- 

 sented nowhere else ; whilst the fishes of other families, 

 later immigrants, all belong to widely distributed genera, 

 and several of them even to species also found either in 

 the Nile or in the Congo, or in both these river-systems. 



The other theory is that the Cichlids have originated 

 as fresh-water fishes in Eocene times in America and have 

 crossed the .Atlantic by a bridge which then connected 

 .South America with Africa. This is the explanation given 

 by Dr. Pellegrin. He admits that we have no indication 

 of any near allies of these fishes before the Middle Eocene 

 (Green-River beds of North .America), and, basing his 

 statement on the last edition of Prof, de Lapparent's 

 " Traits de Geologic " (1900), he says it seems to be beyond 

 doubt that during the Lutetian period, which immediately 

 followed that at w-hich the earliest Cichlids were known 

 to live in the fresh waters of America, a vast continent 

 extended between South .America and .Africa. Should this 

 have reallv been the case, the question of the distribution 

 of the Ciciilids could be regarded as settled. But I cannot 

 satisfy myself that there is any geological evidence to 

 support this view. 



This third hypothesis has this advantage over the two 

 others, that it does not postulate any land-areas in late 

 Eocene or Miocene times, for which there is at present no 

 sufficient evidence, nor a pras-Tertiary and marine origin 

 for the family Cichlids, which is wholly improbable and 

 receives no support from palaeontology. 



On the other hand, it is undeniable that the hypothesis 

 of a South .Atlantic land communication in the Eocene has 

 much in its favour, and when this is really established 

 all diflicultv in explaining the distribution of the Cichlida; 

 will have disappeared. In the meanwhile, to use an 

 appropriate metaphor, we must not construct bridges with- 

 out being sure of our points of attachment, otherwise they 

 are liable to collapse as geological knowledge progresses. 



The Mastacembelid.^. — -At present we are acquainted 

 with thirtv-eight species of Mastacembelus : fourteen from 

 the Indo-Malav region, one from Syria and Mesopotamia, 

 and twentv-three from Tropical Africa. The distribution 

 of these fishes, the fossil remains of w-hich are still un- 

 known, has probably once been a continuous one, climatic 

 and hydrographic conditions possibly accounting for the 

 present discontinuity. We have no data from which to 

 decide whether the Mastacembelids first appeared in Asia 

 or in .Africa, or simultaneously in both parts of the world, 

 as is quite possible on the assumption that the family 

 originated in the Eocene sea extending from the Western 

 Soudan to India. 



NO 1869, VOL. 72] 



This concludes our review of the aflinitics and past 

 history of the principal fresh-water types which characterise 

 the present African fish-fauna. We have endeavoured to 

 show that a Tertiary land connection between .Africa and 

 South -America is not absolutely necessary to explain the 

 many points of agreement between the fresh-water fishes 

 of these two parts of the world, as has been postulated by 

 many writers. Besides, there are still some w-ho hold, as 

 does Prof. G. Pfeffer — whose interesting essay on the zoo- 

 geographical conditions of South America, from the point 

 of view of lower vertebrates, appeared after this -Address 

 had been written — that a former subuniversality of distri- 

 bution will afford a solution to many of these problems 

 without necessitating such a land-connection, as exemplified 

 by the past distribution of the Pleurodiran Chelonians. 

 In this review we have summarised many previous hypo- 

 theses and added a few, but in every case with a feeling 

 of dissatisfaction, fully realising, as we do, the futility 

 of speculations in the present state of the two great 

 branches of knowledge, geology and palceontology, on which 

 the solution of these questions must ultimately rest. 



We mav now pass on to the realm of facts, and survey 

 in the briefest manner the waters of the great continent 

 as they appear after the many discoveries which have of 

 late so greatly increased our knowledge of the -African 

 fishes. 



In the present state of our knowledge of the fresh-water 

 fishes -Africa may be divided into five sub-regions, the 

 discussion of the further subdivision of which would exceed 

 the limits of this -Address : — 



(i) The North-Western Sub-region, or Barbary, and the 

 Northern Sahara, properly forming part of the Pala?arctic 

 region. 



(2) The Western-Central Sub-region, with all the great 

 rivers and lakes, extending to the Nile Delta and the 

 mouth of the Zambesi, for which the term Megapotamian 

 Sub-region has been suggested to me by Dr. Sclater. 



(3) The Eastern Sub-region — .Abyssinia, with the upper 

 tributaries of the Blue Nile, and the countries east of the 

 Rift \'alley and north of the Zambesi. 



(4) The Southern Sub-region — all the waters south of the 

 Zambesi svstem. 



(5) Madagascar. 



The smaller islands of the Indian Ocean have a fresh- 

 water fish-fauna so insignificant that they may be entirely 

 neglected in a broad division of the African region. 



I. The North-Western Sub-regiox. 



In its deficiency in rivers of permanent flow Barbary 

 has much in common with South -Africa, and these two 

 parts of Africa in their fish-fauna present a somewhat 

 analogous example to that on which the now exploded 

 theory of bipolarity was founded. Swelling to foaming 

 torrents in the rainy season or after a storm, reduced to 

 series of pools connected by tiny streams at other times, 

 the watercourses are evidently unsuited to fish-life ; and 

 it is not surprising that, apart from a certain number of 

 forms adapted to live in stagnant, often strongly saline, 

 waters, the fishes should be so few in kind. But they 

 make an interesting assemblage, in which it is easy to 

 discover forms unmistakably suggestive of the pra?-Pliocene 

 times when the sea had not burst through the Straits of 

 Gibraltar, mixed with others of decidedly Africo-Indian or 

 Oriental affinities. 



The number of species from inland waters, whether fresh 

 or salt, hitherto recorded from this part of .Africa, amounts 

 to thirty or thirty-one only. Of these thirteen arc 

 Cvprinids, which may all be regarded as of northern or 

 eastern immigration. Four of the Barbels show European 

 aliinities, one of them being found also in Spain, whilst 

 the seven others belong to a section of the genus largely 

 represented in Southern .Asia and East -Africa, but only 

 known in West Africa from Cameroon. -A species of 

 \'aricorhinus, recently discovered in Morocco, has similar 

 affinities, the genus being known from South-Western -Asia, 

 .Abyssinia, and Lake Tanganyika. A small somewhat 

 aberrant species of the South-Western .Asian genus 

 Phoxinellus has been described from the Algerian Sahara, 

 whilst an Alburnus from the Tell (originally placed in the 

 genus Leuciscus) is also the sole representative in -Africa 

 of a genus inhabiting Europe north of the Pyrenees and 



