298 



THE RISE OF ANIMAL LIFE 



Fig. 13-22. Life history of the frog. 



Hon has occurred as has been described on 

 these pages. 



The frog: the halfway vertebrate 



If numbers be taken as a criterion for 

 success, the amphibians were much more 

 successful at an earher time than now. At 

 that time they did give rise to very success- 

 ful groups of animals, the reptiles, birds, 

 and mammals. The amphibians seem to 

 have reached the halfway mark between the 

 aquatic and the land forms and for that 

 reason they show some very interesting 

 intermediate structures. To study a bird 

 or a mammal without reference to the frog 

 would be like studying the present-day 

 government of the United States without 

 recourse to the struggle for independence. 

 Understanding of a mammal can only come 

 from a historical approach to the whole 



problem, which means that it is essential 

 to examine an intermediate type. There is 

 no better form to use for such a study than 

 a representative amphibian, and the frog 

 lends itself especially well for several rea- 

 sons. First, aside from its well-developed, 

 atypical, jumping legs, it possesses most of 

 the typical ancestral amphibian character- 

 istics. Secondly, it occurs universally, which 

 makes it an inexpensive form for study. 

 Lastly, it is of such a size that it is easily 

 handled in the laboratory by students; a 

 larger or smaller animal offers some difficult 

 problems in this respect. A thorough knowl- 

 edge of the "halfway" animal at this point 

 provides the background for a better un- 

 derstanding of the mammal. 



Life history (Fig. 13-22). One of the first 

 harbingers of spring is the familiar croaking 

 of the frogs. The one heard most frequently 



