52 SECRETS OF ANIMAL LIFE 



except, of course, when in the torpid state into which 

 they sink in winter, when the reduction of vitality 

 minimizes the demand for oxygen. 



There is much in the familiar sequence of events 

 that is extraordinarily interesting, such as the re- 

 capitulation of racial evolution on the one hand 

 and the specificity on the other. Thus there is no 

 doubt that the young tadpole has a two-chambered 

 heart like that of most fishes and a piscine type 

 of circulation. This reads like recapitulation. Yet 

 the tadpole's two sets of gills are quite different 

 from those of ordinary fishes, and its skin is an 

 amphibian's skin from first to last. This is specifi- 

 city. There is a pretty point in regard to the 

 tongue, which is at first non-mobile, just as is the 

 case in fishes. Gradually muscle-fibers increase in 

 the tadpole's tongue, and the foundations are laid 

 of the highly developed musculature that enables 

 the frog to shoot out its tongue in a somersault 

 on the unwary fly. Another general fact of in- 

 structive value is the succession of varied solutions 

 of the same fundamental problems. The sequence 

 of diverse modes of respiration is remarkable. The 

 newly-hatched larva breathes cutaneously; then 

 three pairs of so-called external gills develop; then 

 the mouth is indimpled and gill-clefts open out 

 from the pharynx to the exterior; then a gill- 

 chamber is formed and a second set of gills replaces 

 the first; then gills and lungs are used at the same 

 time, just as in Dipnoan mud-fishes; finally the small 

 fully-formed frog is a lung-breather, with its skin 



