August 24, 1905] 



NA rURE 



417 



isolated lake of volcanic origin on the Cameroon moun- 

 tain, a lake 200 feet above sea-level, without any outlet, 

 and situated about twelve miles from the nearest river 

 and twice as far from the sea-shore. This lake was found 

 to have a fish-fauna consisting exclusively of Cichlids, 

 belonging to three genera and five species, two of which 

 have been described as new. 



The great bulk of the family inhabits Africa, including 

 Madagascar, and America, from Texas to Montevideo ; the 

 number of generic types is greater, although the species 

 are only slightly in excess, in the former than in the latter 

 part of the world. Seven species inhabit Syria, three of 

 these being also found in the Nile, and three are known 

 from India and Ceylon. The American and Indian genera 

 are all distinct from the African. A great number of 

 species (fifty-five), all but one endemic, inhabit Lake 

 Tanganyika, of which they form a little more than two- 

 thirds of the fish-fauna ; and many of these species belong 

 to distinct genera, showing specialisation to a remarkable 

 degree. Out of thirty recognised genera of African 

 Cichlids, as many as fifteen are believed to be peculiar to 

 Tanganyika. Lake Nyassa, with the Upper Shir^, 

 possesses also some remarkable endemic genera ; but they 

 are only four in number, and the number of species re- 

 corded up to the present does not exceed twenty-two. The 

 rest of the species are mostly from West Africa and the 

 Congo basin : but a few, referable to the two most widely 

 spread genera, are found in East and South Africa. 

 Madagascar has only four species, two belonging to an 

 endemic genus, whilst each of the two others is referred 

 to a widely distributed African and Syrian genus. 



No fossils are known that agree closely with any of the 

 recent genera, but a type of Perciforms, described by Cope 

 as Priscacara, from Middle Eocene fresh-water beds in 

 North .\merica, presents all the characters which we should 

 expect to find in the direct ancestors of the modern Cichlids, 

 differing from the living forms in the presence of vomerine 

 teeth, a serrated prae-operculum, and apparently eight 

 branchiostegal rays. It has twenty-four vertebrae, a 

 number lower than is found in most of the recent genera ; 

 and this indication is of importance for reasons that must 

 be explained somewhat fully. 



The lower Teleosteans (Malacopterygii and Ostariophysi, 

 often united under the term " Physostomi ") mostly have 

 a high number of vertebras ; but when we pass on to the 

 higher Acanthopterygii, we find very frequently, among 

 most diverse families, the number reduced to twenty-four. 

 That this number should occur with such frequency has 

 struck many ichthyologists since Dr. Giinther first directed 

 attention to it, more than forty years ago, pointing out 

 at the same time that in the Labridae this number is 

 almost constant in the tropical genera, whilst those genera 

 which are chiefly confined to the temperate seas of the 

 northern and southern hemispheres have an increased 

 number. It has since been shown by Dr. Gill and by 

 Prof. Jordan that this generalisation holds true of several 

 other families of Acanthopterygians, and the latter 

 authority, when discussing the subject at some length, 

 came to the opinion that the state of things could be e.x- 

 plained, from an evolutionary point of view, on the assump- 

 tion that competition among various marine fishes being 

 greater within the tropics has resulted in greater special- 

 isation, by which the originally high number of vertebrje 

 has been reduced. It is dif^cult, however, on this assump- 

 tion to account for the fact that in so many cases the 

 reduction should have resulted in the number twenty-four 

 — neither one more nor one less — and this repeated in many 

 families belonging to the same sub-order but otherwise 

 only remotely related to one another. Three years ago, 

 when dealing with the affinities of the flat-fishes, Pleuro- 

 nectida;, I was struck by the discovery that, in the un- 

 questionably least specialised genus, Psettodes, the vertebrse 

 are twenty-four in number, the other known genera having 

 from twenty-eight to sixty-five, and that the numbers in- 

 creased along the most probable lines of evolution. A 

 consideration of other families, and of the fossil forms in 

 which the number of vertebra; has been ascertained, soon 

 convinced me that this rule also applies to them, and that 

 the order of evolution had in every case to be reversed 

 from that assumed by Prof. Jordan, whose interpretation 

 I had previously accepted as correct. As a result of my 



investigation into this question I believe that the frequent 

 occurrence of twenty-four vertebrae is due to the original 

 Acanthopterygians having presented this number, that it 

 has been retained in the more generalised members of the 

 families which have branched off from them, and increased 

 or, more seldom, reduced in the course of evolution. 



The view which I entertained when first studying the 

 Cichlids of Lake Tanganyika must be abandoned, and the 

 direction of the supposed lines of evolution reversed, 

 together with the signification given by me to the 

 characters of increased number of dorsal and anal rays, or 

 of multiple lateral lines which go more or less hand in 

 hand with the increase in the vertebral segments. I must 

 therefore repudiate the statement, first made by me in 

 describing some of the new genera discovered by Mr. 

 Moore in Lake Tanganyika, that they show features of 

 generalisation, the contrary being the case. This has been 

 shown by Dr. J. Pellegrin, who has recently published a 

 monograph of the whole family Cichlidae, in which he has 

 very ably dealt with the question of the interrelation of 

 the various genera from the phylogenetic point of view. 



Two theories have lately been put forward as to the 

 origin of the African Cichlids. 



According to Mr. Moore, to whom we owe the discovery 

 of so many new forms in Lake Tanganyika, the Cichlids 

 are of marine origin, and penetrated into a hypothetical 

 Central African sea in prse-Tertiary times. But as no 

 Perciform fish of any sort is known earlier than the Upper 

 Cretaceous, and no Perch, in the widest sense, before the 

 Lower Eocene (Prolates), the possible existence at that 

 remote time of so specialised a type of Perches as the 

 Cichlids is absolutely contrary to palKontological evidence. 

 Further, such an explanation is unsupported by any geo- 

 logical data, no trace of Jurassic or Cretaceous deposits 

 having been found on the plateau of Central Africa, not- 

 withstanding much search over a considerable portion of 

 the Congo State. It is impossible to imagine that such 

 a sea could have existed without leaving any sedimentary 

 deposits whilst its relics were being preserved in Lake 

 Tanganyika. Besides, the distinguished Belgian geologist. 

 Prof. J. Cornet, who has paid special attention to this 

 question, and has himself surveyed a considerable part of 

 the territory of the Congo State, regards the Tanganyika 

 as by no means a very ancient lake, its formation not 

 dating back beyond Miocene times. I may also here point 

 out that Mr. Moore's interpretation of the affinities of the 

 so-called " halolimnic " ^Iollusca has not received any 

 support from those best able to judge of its merits. Mr. 

 E. A. Smith, from the recent conchological, and Mr. 

 Huddleston, from the pala^ontological point of view, have 

 recently discussed his conclusions, with which they are 

 unable to agree. I need hardly add that the discovery 

 since the publication of the " Tanganyika Problem " of 

 the Medusa Limnocnida tanganicac in Lake Victoria has 

 dealt a further blow to Mr. Moore's theory. 



.As regards the origin of this Medusa, recent palaeonto- 

 logical discoveries afford a much more rational explanation 

 of the presence in Tanganyika of a Ccelenterate of un- 

 questionably marine derivation. The highly important finds 

 of fossils between the Niger and Lake Chad by the English 

 and French officers of the Boundary Commission, which 

 have been reported upon by Prof, de Lapparent, Mr. Bullen 

 Newton, and Dr. JSather, have conclusively established 

 the existence of Middle Eocene marine deposits over the 

 Western Soudan, and the Egyptian and Indian character 

 of these fossils, as well as of others previously obtained in 

 Cameroon and Somaliland, justifies the belief in a Lutetian 

 (Middle Eocene) sea extending across the Soudan to India. 

 In fact, as stated by Mr. Newton, the palaiontological 

 evidence seems to prove that the greater part of Africa 

 above the equator was covered by sea during part of the 

 Eocene period. On this sea retreating northwards, after 

 the Lutetian period, Medusa became land-locked and 

 gradually adapted themselves to fresh water : they had not 

 far to travel to find themselves in what are now the Nile 

 lakes, and later, through the changes which Mr. Moore 

 himself has shown to have taken place in the drainage of 

 Lake Kivu, they were easily carried into the Tanganyika 

 — probably at no very remote time — and maintained them- 

 selves to the present day. I understand that the Medusa 

 reported from Bammaku, Upper Niger, in 1895, t»Jt still 



NO. 1869, VOL. 72] 



