TRANSACTIONS OF SECTION D, 413 
of extreme simplicity, and that a knowledge of the river-systems ought to suffice 
for tracing areas which shall express the state of things. But we must bear 
in mind the movements which have taken place on the surface of the earth, and 
owing to which the conditions we find at present may not have existed within 
comparatively recent times; and this is where the systematic study of the aquatic 
animals affords scope for conclusions having a direct bearing on the physical 
geography of the near past. To mention two examples, the fishes of the Nile 
show so many specific types in common with those of the Senegal-Niger, now 
more or less completely separated by the Chad basin, that we felt justified in 
postulating a recent communication between these water-systems, which has 
been fully confirmed by the study of the Lake Chad fishes; whilst, on the other 
hand, the greater difference between the fishes of the Nile and those of the 
Congo basin, the waters of which interlock at present in such a way that it is 
believed possible, at certain seasons, for a man in a boat to pass from the one 
into the other, points to the existence, until very recently, of a more effective 
separation, Such problems are of the greatest interest, and a more exact 
knowledge of the fishes will help towards their solution. 
There is another aspect of the question of geographical distribution which 
has assumed special importance of late, especially in the writings of Professor 
Osborn, Mr, Lydekker, and Dr. Scharff, and of which Dr, A. E. Ortmann’s paper 
on the distribution of Decapod Crustaceans, published three years ago, may be 
taken as an example. One of the conclusions formulated therein is that ‘any 
division of the earth’s surface into zoo-geographical regions which starts exclu- 
sively from the present distribution of animals without considering its origin 
must be unsatisfactory.’ But in certain groups of animals, possibly in most, the 
question of their origin is not easily settled; in the case of the African fresh- 
water fishes, for instance, we sadly lack all direct paleontological data, such as 
have sprung up lately in marvellous profusion in the case of the mammals, and 
notwithstanding the great progress in our knowledge of the changes that have 
taken place in the configuration of the world in Secondary and Tertiary times, 
which has been conveyed to a wide circle of readers chiefly through the luminous 
works of Neumayr, Suess, and de Lapparent, there is still much that is open 
to discussion. It must be admitted—and it is well to draw special attention to 
this point—that Dr. Ortmann’s maps of the land-areas in past periods, which 
render his suggestive paper so attractive, cannot be accepted as the expression of 
well-established geological facts, and are, in some respects, gravely misleading. 
If I have attempted to deal with this subject on the lines laid down by Dr. 
Ortmann, whilst realising the want of many necessary data, paleontological and 
geological, on which to base conclusions, it is with a due sense of humility, 
being fully aware that the suggestions now offered must be regarded as mere 
speculations. 
The time has come for a stock-taking of our immensely increased material, the 
previous accounts of the distribution of African fishes given by Dambeck in 
1879, by Giinther and by Sauvage in 1880, and by Palacky in 1895, no longer 
answering, even approximately, to our present knowledge, as may be seen by 
comparing the lists given by these authors with the one I have quite recently 
published in the ‘ Annals and Magazine of Natural History’ as a basis for the 
- sketch here attempted. 
How little we knew of the fresh-water fishes of Africa when the subject was 
dealt with by the above-named authors is exemplified by the enormous number 
of genera and species which have been discovered within the last few years, 
thanks chiefly to the enlightened activity of the Governments of Egypt and the 
Congo Free State, and to the initiative of Professor Ray Lankester in organising 
explorations of the great lakes of Central Africa. The waters of the French 
Congo and Cameroon, the Niger, Abyssinia, and the interior of East Africa, have 
also yielded a large number of novelties; even the Nile, comparatively so well 
known, has been productive of many and remarkable additions to our knowledge. 
The importance of a better acquaintance with the fishes of the Lower Nile, a 
district believed to have been particularly well explored, can be measured by 
