INTRODUCTION. Xlll 
fresh-water fishes belong to families which are wholly (eg. Percidew, Cichlide) 
or chiefly (e.g. Cyprinodontide, Siluride) confined to fresh water. In Mexico 
and Central America these are the following:—Cichlide, Percide, Centrarchide, 
Cyprinodontide, Loricariide, Siluride, Cyprinide, Gymnotide, Characinidee, and 
Lepidosteide. 
The Cicuuipa are Perciform fishes which bear a considerable resemblance in 
appearance and anatomy to the most generalized group of Perciformes, the widely- 
distributed marine family Serranide, but differ from them in certain features of 
specialization, such as the presence of but a single nostril on each side, the absence 
of teeth on the palate, the coalescence or sutural union of the lower pharyngeals, and 
the reduced number of branchiostegal rays. 
No known fossils can be referred to the Cichlide, which inhabit America, from 
Texas to Montevideo, and Africa, including Madagascar. Seven species occur in 
Syria, and a single genus with three species inhabits Ceylon and Southern India. 
The American Cichlidea comprise over 150 species which may be arranged in 
23 genera; from Africa more than 200 species referred to 35 genera have been 
described. 
Not one of the genera is common to Africa and America, but the South-American 
Acara is scarcely generically distinct from the African Paratilapia, and there can be 
no doubt that these are the most generalized of living Cichlide and very near to the 
ancestral type of the family. 
The Mexican and Central-American Cichlide are more specialized than the South- 
American ones, and have certainly been derived from them; not one of the genera 
with three anal spines is found north of the Isthmus of Panama, and all the South- 
American Cichlide have simple conical teeth. 
Of the Southern types only Cichlosoma has reached Mexico and Central America, 
and has there given rise to a variety of more specialized forms. 
We have no evidence in favour of dating the origin of the Cichlide before the 
Eocene. At the same time we have to explain their occurrence in South America, 
Africa, and India at the present day. . Boulenger, in his address to the Zoological 
Section of the British Association in 1905, whilst adopting a non-committal attitude, 
put forward the hypothesis that the Cichlide were originally a Northern group, and 
that in the Kocene they ranged over North America and Northern and Eastern Asia, 
which were then one continent, and that they have attained their present distribution 
by a southward migration and by becoming extinct in their original habitat. This view 
