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 
