74 ANIMAL LIFE AND HUMAN PROGRESS 



they have been properly investigated, it has been found that 

 these species differ in habits. Sometimes such species may 

 differ markedly in adaptive characters but show close agree- 

 ment in all other features. 



Two species of fishes of the Herring family of a type 

 peculiar to Lake Tanganyika are so similar in appearance and 

 in nearly all other characters, even in details of coloration, as 

 to leave no doubt as to their very close relationship ; indeed, 

 they were originally described as one species, Pellonula miodon ; 

 nevertheless I have shown that these two forms differ in a 

 most remarkable manner in the structure of the mouth and 

 in their dentition. 



The Cichlidae are a family of Perch-like fresh-water fishes 

 in which the lower pharyngeals, a pair of bones in the throat 

 that bear teeth, are joined together. Two species of this 

 family from Western Ecuador are so like each other that they 

 were originally described as one, Herosfestae. But in one, as 

 in several related species found elsewhere, the lower jaw pro- 

 jects, the teeth in the jaws are sharp, the pharyngeal teeth are 

 conical and the pharyngeal bones are merely coalescent by 

 their straight inner edges to form a triangular plate of moderate 

 size ; in the second species the lower jaw is shorter than tjbe 

 upper, the jaw teeth are blunt, the pharyngeal teeth form a 

 flat pavement and the bones that bear them are united by a 

 deeply interlocking suture to form a broad and massive plate. 

 It is clear that the first species is predaceous, and I have 

 proved that the second feeds on molluscs by finding the stomach 

 full of broken shells. I believe that a part of the ancestral 

 stock gave up chasing little fishes and took to eating shell-fish 

 on the bottom, and that this was the initial step in the 

 differentiation of the two species. 



I will now pass on to some more striking instances of adap- 

 tive evolution. The Cyprinidae the Carp family are fresh- 

 water fishes of Europe, Asia, Africa and North America ; the 

 mouth is toothless, but the pharyngeal bones, in the throat, 

 bear teeth that bite upwards against a horny pad supported by 

 the base of the skull, and these teeth vary in form, number 

 and arrangement, according to the nature of the food. 



