CHAPTERS ON EVOLUTION. 



to in a previous chapter in connection with the evolution of lungs. 

 It is needful, however, again briefly to refer to this life-history as a 

 starting-point for the due understanding of other and allied cases of 

 development. The frog begins its existence as a tadpole (Fig. 159), 

 breathing first by external and then by internal gills, and possessing a 

 two-chambered heart, resembling that of the fish. Sooner or later 

 the hind limbs begin to appear, then the fore limbs are developed, 

 and the frog's lungs likewise begin to make their appearance. At 

 this stage, the animal resembles its neighbours, the Proteus and 

 Axolotl (Fig. 1 60), which are tailed, and which breathe throughout 

 life by both gills and lungs. Later on, the gills disappear entirely ; the 



tail becomes rudiment- 

 ary; and the frog, leav- 

 ing the water, becomes 

 the terrestrial lung- 

 breather with which 

 we are so familiar. 

 To repeat Huxley's 

 words in reference to 

 the case for develop- 

 ment as a guide to the 

 history of the race : 

 "If all living beings 

 have come into exist- 

 ence by the gradual 

 modification, through 

 a long series of genera- 

 tions, of a primordial living matter, the phenomena of embryonic 

 development ought to be explicable as particular cases of the 

 general law of hereditary transmission. On this view a tadpole is 

 first a fish, and then a tailed amphibian, provided with both gills 

 and lungs, before it becomes a frog, because the frog was the last 

 form in a series of modifications whereby some ancient fish 

 became an urodele (or tailed) amphibian, and the urodele amphibian 

 became an anurous (or tailless) amphibian. In fact, the develop- 

 ment of the embryo is a recapitulation of the ancestral history of the 

 species." That there are "ancient fishes," still represented by living 

 species, which may have served as the starting point of the frog-race, is 

 matter of zoological fact. 



" Various features in the anatomy of the Tadpole," says the late 

 Professor F. M. Balfour, " point to its being a repetition of a primi- 

 tive vertebrate type. The nearest living representative of this type 

 appears to be the Lamprey." This author also points out how close 

 are the resemblances between the mouths of the tadpole and 

 lamprey ; and a still more remarkable fact consists in the observation 



FIG. 160. AXOLOTL, SHOWING THE EXTERNAL GILLS. 



