224 THE FROG CHAP. 



belonging to the various geological formations a com- 

 plete series of animal remains, representing all the 

 stages in the evolution of the highest from the lowest 

 forms. But owing to various causes which we cannot 

 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 pal&ontology 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 metamorphosis into the more 

 highly organised frog. According to the theory of 

 , 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 determine 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 descended from ancestors with horny 

 jaws and without teeth, though in all probability these 



