224 THE FROG chap. 



ing all the stages in the evolution of the highest from the 

 lowest forms. But owing to various causes which we can- 

 not further consider here, the record of the succession of 

 life on the globe is very imperfect and incomplete in the 

 case of any individual species : but the evidence furnished 

 by Palaeontology — as the study of fossils is called — is very 

 important in supplying proofs that there has been a gradual 

 evolution of the higher from the lower forms. 



Further evidence in the same direction is furnished by 

 Morphology and Embryology, as will be more apparent to 

 you when you have studied a number of other kinds of 

 animals. You have, however, seen that the frog begins 

 life as a single cell, and that it is possible to trace a series of 

 modifications which gradually convert the unicellular oosperm 

 into a tadpole — which is to all intents and purposes a fish, 

 and that the tadpole subsequently undergoes metamor- 

 phosis into the more highly organised frog. According to 

 the theory of recapitulation, these facts indicate that the 

 frog repeats, during its single life, the series of changes 

 passed through by its ancestors in the course of ages. In 

 other words, ontogeny, or the evolution of the individual, 

 is, in its main features, a recapitulation of phylogeny, or 

 the evolution of the race. At the same time you must 

 bear in mind that it is not always an easy matter to deter- 

 mine which characters are of phylogenetic significance, and 

 which have been secondarily acquired owing to various 

 causes. To take an example : — the horny jaws of the 

 tadpole might be taken to indicate that frogs were de- 

 scended from ancestors with horny jaws and without teeth, 

 though in all probability these organs have been secondarily 

 acquired as adaptations in connection with the habits of the 

 tadpole, and have therefore, unlike the gills, for instance, 

 no ancestral significance. 



