176 THE DARWINIAN THEORY 



The inevitable conclusion is that, of the five 

 groups of Craniate Vertebrates, fish are the most 

 primitive ; and that the other four groups are 

 descended, if not from fish, at any rate from gill- 

 breathing forms, aquatic and presumably fish-like. 

 The evidence afforded by Palaeontology does not 

 help us much on this point, the oldest known Verte- 

 brates being fishes from the lower Silurian deposits. 

 Geology would favour an aquatic origin for Verte- 

 brates as for other forms, the instability of the land 

 in comparison with the sea being well known. 



Amphibia. — The chief point of interest in Am- 

 phibia is the well-known transitional series from the 

 gill-breathing to the lung-breathing condition, which 

 we have already dealt with at length when discussing 

 the theory of change of function. Almost all Am- 

 phibia commence life as gill-breathers ; but in one 

 or two cases, such as Hylodes, where the eggs are 

 not laid in water, owing to abundance of food-yolk 

 the early stages are passed through before hatching, 

 and, as in the higher Vertebrates, the gill-breathing 

 stage is dropped out, though gill-clefts are developed 

 (see Fig. 16). 



The pedigree of Amphibia may be regarded as 

 established, and is as follows : Amphibia are de- 

 scended from fish, which migrated into rivers to 

 avoid enemies and to obtain food. Owing to 

 drought the air bladder became converted into a 

 lung, and this at once conferred the power of going 

 on land (see Fig. 29). This led to the conversion 

 of a fin into a definite pentadactyle limb, a change 

 the true nature of which is uncertain. The point of 



