RECAPITULATION 225 



growth and development of all Vertebrates, and we 

 are only beginning to understand the effects of 

 defect or excess of this secretion. There is nothing 

 very surprising in the fact that excess in the case 

 of the Axolotl causes the occurrence of the meta- 

 morphosis which had already in numerous experi- 

 ments been produced by forcing the animals to 

 breathe air. 



Metamorphosis, as in the development of gill 

 arches and gill slits in the embryos of Birds, Reptiles, 

 and Mammals, exhibits a recapitulation of the stages 

 of evolution of certain organs. But in the case 

 of other organs the absence of recapitulation is 

 remarkable by contrast. If, as I believe, the de- 

 velopment of lungs and disappearance of gills was 

 directly due to the necessity of breathing air, it is 

 difficult to avoid the conclusion that the terrestrial 

 legs were originally evolved from some type of fishes' 

 fins by the use of the fins for terrestrial locomotion. 

 Yet neither the amphibian larva nor the embryo 

 of higher Vertebrates develops anything closely 

 similar to a fin. There is no gradual change of a fin- 

 like limb into a leg, but the leg develops directly 

 from a simple bud of tissue. The larva of the 

 Urodela is probably more primitive than the tadpole 

 of the Frogs and Toads, and in the former the legs 

 develop while the external gills are still large, long 

 before the animal leaves the water. 



It is possible that the limbs were transformed to 

 the terrestrial type before the animal itself became 

 terrestrial, the habit of swimming having been 

 partly abandoned for that of crawling or walking 

 at the bottom of the water, and the tail being used 

 merely for swimming to the surface to obtain air. 



p 



