THE EVIDENCE FROM DEVELOPMENT. 237 



that many of the peculiarities of the skull of the tadpole are repro- 

 duced in the skull of the lamprey. Whilst, as Professor Balfour 

 remarks, these resemblances must be due to deeper causes than 

 mere adaptation to similar habits, there are yet "no grounds for 

 supposing that the lamprey itself is closely related to an ancestral 

 form of the Amphibia." The more feasible opinion is that which 

 would assume that both lamprey and tadpole are descended from a 

 common and still more primitive stock. It would seem, indeed, that 

 the ganoid fishes (of which the sturgeon, bony pike, and the fossil 

 forms of the Old Red Sandstone, are examples) together with the 

 allied Lepidosiren and Ceratodus (see page 113, et se<?.), possess an 

 allied origin from the common root of lamprey and tadpole. The 

 facts already detailed (see Chapter VI.) concerning the development 

 of lungs in these latter fishes, would of themselves seem to support 

 this idea of their origin. Just as along one line of descent, the gilled 

 tadpole-race has developed its lungs as the frogs of to-day, so, start- 

 ing from the same ancestry, but developing along a different pathway, 

 the lepidosirens and their neighbours, as if animated by like air- 

 breathing tendencies, have developed lungs likewise. Similar ten- 

 dencies towards a certain goal in development, in other words, have 

 produced like results in the evolution of two branches of the 

 vertebrate tree. 



An interesting fact may be added to these considerations, namely, 

 that one tadpole-form, that of Dactykthra, presents the closest re- 

 semblance to certain fossil fishes of the Old Red Sandstone period. 

 These fishes belong to the ganoid type, already mentioned, and the 

 Dactylethra tadpole, whilst resembling the ganoid stock, also shows 

 affinities to the shark and dog-fish type. The latter fishes are nearly 

 allied to the ganoids, and both appear in the geological record at 

 once as the oldest of fossil fishes and vertebrates. This curious tad- 

 pole probably represents a period in the evolution of the frog-race, 

 after that race had been specialised from the lamprey stage, and had 

 already advanced somewhat on its amphibian pathway. 



It therefore requires no stretch of the imagination, but the exercise 

 of sober reason, to note, firstly, that as all the amphibian class frogs, 

 toads, newts (Fig. 161), and their less familiar neighbours tailed 

 and tailless, together begin life as tadpoles ; and, secondly, that as 

 they end, some like the frogs, tailless and gill-less, others like the 

 proteus or axolotl (Fig. 160), possessing both gills, lungs, and 

 tails, the assumption remains clear that these animals have sprung 

 from a fish ancestry. It is further matter of fact that their develop- 

 ment has followed two pathways. In the one case the frogs and 

 toads have passed towards a pure air-breathing existence, and have 

 emerged from their development as land animals, pure and simple. 

 In the other case the lower stock of the class, represented by the 



