92 Mutations and Evolution. 



limbs in diverticula of the branchial chambers, and such special 

 adaptations as the ventral disc or sucker of certain mountain forms." 

 The existence of such adaptational characters is supported by the 

 fact that certain species differ more widely in their tadpoles than in 

 their adult stages. Thus (Boulenger and Annandale, 1918) the Indian 

 species Rana tigrina and R. cancrivora are so similar as frogs that 

 the latter was classed as a variety of the former. But Annandale 

 (I.e.) has shown that the tadpoles differ markedly in buccal armature. 

 Whether this is a case of convergence of the adult species, as 

 Dr. Annandale thinks, or divergence of the tadpoles, as Dr. 

 Boulenger believes, need not concern us here, though from analogy 

 the latter interpretation appears more probable. 



To return to the recapitulatory characters, the transformation 

 of the fish-like gill-arches of the tadpole into the aortic arches of 

 higher vertebrates is too well known to require comment here, except 

 to point out that it comes about through the gradual substitution 

 of one series of blood vessels for another, the branches to the gills 

 being gradually pinched off and the blood stream diverted to the 

 more direct route to the lungs. The specious argument that the 

 development of any recapitulatory character must go through such 

 preliminary stages for purely structural developmental reasons is 

 now seldom heard and can very well be consigned to oblivion. 

 Developmental mechanics as well as comparative embryology tell 

 strongly against it. 



The experiments of Gudernatsch (1914) in greatly retarding or 

 hastening the time of metamorphosis by feeding tadpoles on thymus 

 or thyroid respectively, showing that growth and differentiation are 

 separate factors, do not affect our present interpretation. They 

 merely indicate that the processes of development and metamorphosis 

 are physiologically controlled by something in the body of the nature 

 of hormones or enzymes secreted by certain tissues. A recent 

 paper (Morse, 1918) has attempted a further analysis of the 

 processes that lead to atrophy of the tail in metamorphosis. The 

 writer concludes that autolysis is the primary physiological factor. 

 The first step in atrophy, according to Barfurth, is the growth of 

 the pygostyle which, by occlusion of the blood vessels in the tail, 

 causes an accumulation of CO 2 and acids. This acidosis of the 

 tissues induces autolysis. In this condition the phagocytes are 

 chemotactically attracted to the atrophying organs, so that 

 phagocytosis is a secondary, and not the primary factor as 

 Metschnikoff supposed. Hormones or enzymes probably stimulate 



