244 haeckel's anthropogenie: 



diverging brandies, from the first of wMcli the teleostei were derel- 

 oped in the mesozoic period ; from the latter, the amphibia, in the 

 Devonian period. In the ancestral series, then, the selachia constitute 

 the eleventh stage, the dipnoi the twelfth, and th-e amphibia the 

 thirteenth. The living selachia and dipnoi are mere remnants of 

 formerly" much developed groups. The dipnoi represent a consider- 

 able advance on the selachia from the change of habit of life, for the 

 dipnoi are true amphibious animals according to the ordinary sense 

 of the word. This advance shows itself in the modification of the 

 air-bladder into a lung, the communication of the nasal cavity with 

 the mouth, and the development of three chambers to the heart. 

 Among living dipnoi, ceratodus is an especially conservative and 

 primordial form, as marked by the skeleton of its fin and single lung, 

 yet the three living forms with which we are acquainted must have 

 difiered widely from the common ancestral form. The dipnoi form 

 a stage exactly intermediate between the fish and the amphibian. 



None of the existing amphibia can be looked upon as representing 

 the ancestral form from which sprung the higher vertebrate groups, 

 but the group was abimdantly represented by highly developed forms 

 in the carboniferous period. The advance on the dipnoi is especially 

 marked in limbs, and is correlated with their new mode of life. The 

 limbs become transversely segmented, and possess each five toes. 

 This number five has been inherited by all other vertebrata, and 

 when there are less this can always be explained by adaptation : the 

 evidence on this point is so complete that no comparative anatomist 

 can doubt that the higher vertebrates have been developed from a 

 four-limbed and five-toed ancestor, e.g., the relation of equus to 

 anchitherium. 



The amphibia form a particularly interesting group, on account of 

 the fact that several of the lower forms have stopped at the difierent 

 stages of phylogenesis indicated by the ontogenesis of the higher forms } 

 and an acquaintance with the comparative anatomy a>nd ontogeny of 

 the amphibia is sufficient to convince one that man, like all higher 

 vertebrates, is derived from a long-tailed branchiate ancestor, and that 

 only, this hypothesis is sufficient to account for his rudimentary tail 

 and visceral arches and clefts. 



Among the amphibia there may then be established two stages, 

 the thirteenth with persistent gills, and the fourteenth with the tail 

 but without the gUls. 



