74 ZOOLOGY 



young lives in water and strongly resembles a fish, and is 

 then known as a tadpole. If the tadpole be carefully ex- 

 amined it will be found to present strong resemblances in 

 structure to an archaic type of fish known as the lung-fish, and 

 represented now only by three types, one confined to the rivers 

 of Australia, one to the rivers of Central Africa, and one to the 

 rivers of South America. Like the tadpoles these fish possess 

 both lungs and gills, and in the arrangement of their blood- 

 vessels and in the structure of their brains there is likewise a 

 strong resemblance to tadpoles. If we interpret these resem- 

 blances as evidence of blood-affinity we shall be bound to con- 

 clude that frogs have been developed out of lung-fish. Now 

 it happens that there is corroborative evidence from fossils to 

 support this conclusion. In the rocks known as Devonian or Old 

 Red Sandstone there are numerous remains of fish. Many of 

 these, according to the opinion of the best experts, were allied to 

 the lung-fish. In the Carboniferous rocks which lie above 

 the Devonian, in which the coal-seams are found, abundant 

 remains of very primitive frog-like and newt-like creatures have 

 been discovered, and there seems to be a strong probability 

 that frogs and newts, that is, Amphibia, were evolved out of 

 creatures like lung-fish in the interval between the periods 

 represented by these two sets of rocks. But if this con- 

 clusion be accepted the consequence follows that the frog, 

 in its growth from the egg to the adult condition, gives a 

 summary of the history of the race of frogs in their age- 

 long progress from the condition of fish to the condition 

 of land animals. Now this case of the tadpole and the 

 frog is only one of numerous instances of the same sort of 

 thing. Thus, for instance, the oyster is an exception among 

 bivalves, in that it possesses no foot i.e. no axe-like median 

 muscular mass attached to the underside of its body. But 

 the American oyster, when about |~inch long, has such a ( foot, 

 which it loses as it grows up. When young it moves about, 

 but when older it stays in one place. There are small salt- 

 water shrimps belonging to the class Crustacea, which when 

 young swim about freely and resemble other species of the 

 same order, but which when older attach themselves to the 

 gills and mouth of fish and degenerate into shapeless parasites 

 whom no one would suspect of belonging to the class Crustacea 



