184 ANIMAL METAMOEPHOSIS. 



to make the study of natural history, which was formerly little 

 more than a dry catalogue of facts, "at once a speaking history of 

 the past and a prophetic index of the future." Haekel, Spencer, 

 Darwin, and others, have shown us that every animal has been 

 moulded through a wonderful series of metamorphoses into its 

 present shape by surrounding conditions, and that each bears in 

 its parts, or form, the traces, where w^e can read them, of its 

 development or evolution. The study of embryology is difficult* 

 needing large opportunities and great skill in microscopical manip- 

 ulation, but by the study of metamorphosis many of these 

 changes are visible to the unaided eye of the ordinary observer. 



Where a definite larval form — as the Nauplius of the Crusta- 

 cea, or the caterpillar of the Insecta — is common to many 

 members of a sub-kingdom, it seems highly probable that it is the 

 re-capitulative representative of an ancestral form common to all 

 this set of animals, which, if not exactly like a Nauplius or a 

 caterpillar, was not very different from it. 



The permanent condition of the lower members of any divi- 

 sion of the animal kingdom is often represented by the transitory 

 stages of the higher members of the same division. The devel- 

 opment of the frog is a good illustration of this general zoological 

 law. The early condition of the young tadpole when the branchise 

 are external is represented permanently by the adult Proteus or 

 Siren, and the second stage when the gills have disappeared and 

 the limbs have been developed, but the tail has not been wholly 

 absorbed, finds its representative in the newt. Hence we are 

 justified in concluding that the tadpole is a re-capitulative phase of 

 development, and represents more or less closely an ancestor of 

 the frog, which was provided with gills and tail in the adult 

 state, and possessed neither legs nor lungs. In other words, we 

 may infer that, as the adult frog develops from the gilled tadpole 

 through the intermediate tailed stage, so, as a class, the frog-hke, 

 tail-less batrachia developed at a later period out of the tailed 

 batrachia, as the latter had developed out of the gilled batrachia, 

 which originally existed alone. 



Again. Metamorphosis has shown that evolution has not 

 always proceeded towards a gradual development to a greater 

 complexity of form and structure, but that in some instances it 



