130 THE EVOLUTION OF MAN. 



few Tailed Batrachians retain the gill-opening in the side 

 of the neck, though the gills themselves are lost {Meno- 

 poma). If the larv?e of the Salamanders (Fig. 193) and 

 Tritons are compelled to remain in water, and not allowed 

 to get on land, they may, under favourable conditions, be 

 made to retain their gills. In this fish-like condition they 

 become sexually mature, and will throughout life remain 

 compulsorily in the lower stage of development of the 

 Gilled Batrachians. The opposite experiment was made 

 some years ago in the case of the Mexican Gilled Batra- 

 chian, the fish-like AxolotJ (Siredon pisciformis; Plate 

 XIII. Fig. 1). This animal had previously been regarded 

 as a permanent Gilled Batrachian, remaining throughout 

 life in this fish-like condition. But of the hundreds of 

 these animals kept in the Jardin des Plantes at Paris, a few 

 individuals, for some unknown reason, crept to land, lost 

 their gills, and changed into a form closely allied to that of 

 the Salamander (Amhlystoma, Fig. 2). In this state they 

 became sexually mature.^^^ This phenomenon, which at 

 first excited a lively interest, has since been repeatedly 

 observed with care. Zoologists regarded the fact as some- 

 thing peculiarly wonderful, though each spring every 

 common Frog and Salamander passes through the same 

 modification. In these animals we can in the same way 

 follow each step in the significant metamorphosis of the 

 aquatic and gill-respiring animal into the terrestrial and 

 lung-respiring animal. That which thus takes place in the 

 individual during germ-evolution, took place in the same 

 way in the whole class during the course of its tribal 

 history. 



The metamorphosis which takes place in the third order 



