334 Elementary Biology. 



carnivorous animal, for the tadpole is only partly so. This 

 change involves a certain amount of modification in the 

 length and character of the alimentary canal, for that organ 

 in the carnivora is relatively shorter than in the herbivora. 



The entire series of changes thus briefly summarised 

 takes two to six months according to the temperature, being 

 longer in cold and shorter in warm weather. The first 

 fortnight or so of the embryonic life is spent within the egg- 

 membrane 5 the metamorphosis, however, is gone through 

 after hatching in fresh water. 



It will be noted that that metamorphosis consists in the 

 gradual alteration of a herbivorous aquatic animal, breathing 

 of necessity by means of gills, into a carnivorous terrestrial 

 creature, breathing by means of lungs though capable of en- 

 during without injury prolonged immersion. The title of 

 amphibian is therefore fully justified, and the early life-history 

 of the frog thus forms one of the most interesting cases of 

 development known to us, although in many respects 

 special in its character and confined to the grojp of which 

 it is a representative. 



An examination of the embryonic histories of higher 

 types, as we have already hinted (p. 53), shows many inter- 

 esting transition stages, pointing to genealogical relation- 

 ships with a primitive chordate type. The possession of 

 branchial arches and clefts, a tail more or less rudimentary, 

 certain transitory phases in the formation of the circulatory 

 system, and such like, are here alluded to. For a detailed 

 consideration of these and similar phenomena, and indeed 

 for many details with regard to the structure and life-history 

 of the frog itself, reference must be made to zoological text- 

 books. The above sketch of the organisation of the frog will 

 have served its purpose if it show how from such a simple 

 typical form as Amphioxus it is possible to derive an or- 

 ganism as complicated as that which we have been consider- 

 ing. We shall find in the next and concluding section that 

 the main principles we have already laid down are true, not 



