174 PRIMITIVE FORMS OF LIFE 



possesses at birth practically every character of its permanent 

 form except the wings and remains active while these are 

 forming, instead of passing through that crisis of immobility 

 and renovation constituting the metamorphosis of the more 

 recent forms. 



The prairies and forests also became full of life. But, at the 

 same time, another phenomenon of the greatest importance 

 occurred — the invasion of the land surface by the Vertebrates. 

 The starting-point in this great line of organic evolution, 

 naturally enough was the Fish ; two things were necessary 

 for its progress. Firstly, an important modification of the 

 respiratory apparatus, to protect it from the dangers that might 

 result from variations in the hygrometric condition of the air, 

 and secondly, the transformation of the fins into feet capable 

 of treading dry land. We cannot say which particular fossil 

 Fish manifest the first stages in the development of lungs, 

 though certain of the existing forms may perhaps give us some 

 idea of what took place. These are older types, such as 

 Polypterus and Protopterus of the African rivers, Lepidosiren 

 of America, and Neoceratodus of Australia, or relatively modern 

 forms like the Siluridse of the genera Heterobranchus, and 

 Saccobranchus inhabiting the Nile. All these freshwater Fish 

 live in large rivers subject to floods, which make the water 

 extremely muddy, and their respiration is greatly hindered 

 during the period when the water is polluted, so that they are 

 obliged to have a continual current of blood passing rapidly 

 through their branchiae in order to keep them active. The 

 branchiae are therefore more abundantly nourished. Their 

 epidermis develops more rapidly and ends by bearing 

 branching processes constituting supplementary branchiae. 

 These arborescent growths are highly developed in certain Fish 

 of the Siluridan order, the Heterobranchs and the Clarias, 

 and they are contained in a pouch-like expansion of the 

 branchial cavity. Unquestionably the external branchiae 

 observed in the early stages of Polypterus, and which, at all 

 events, are represented in the Ganoids, correspond to these 

 arborescent processes. They are very apparent in the young of 

 the Dipnoid order. Moreover, the expansion of the branchial 

 cavity of the Heterobranchs into the pouch-form in turn 

 corresponds in its vascular details to the pair of long pouches 

 attached to the branchial cavity in those other Siluridae, the 



