122 LECTUEES ON BIOLOGY 



quently long thought that they belonged to a type with per- 

 sistent gills. But great was the surprise when a few individuals, 

 born in the aquarium of the Botanical Gardens at Paris, were 

 observed to lose their gills, leave the water, and become typical 

 terrestrial salamanders. 



There are, however, two explanations possible : it may be 

 that we have here either a tenacious clinging to a low state of 

 development, or a reverting to an earlier stage, consequent upon 

 changed conditions of life. This latter view is upheld by "Weis- 

 mann who thinks that ancestors of the Axolotl which inhabits 

 to-day the Mexican lakes were already in the diluvial period 

 fully developed salamanders. As a result of deforestation and 

 consequent decrease of moisture of the air the animals would 

 have been doomed to extinction if they had not been able to live 

 once more in the water by reverting to the gill- stage. However 

 that may be, they offer in any case a proof of the variability of 

 organic forms. 



It is a rule almost without exception that the vertebrates 

 possess a shape which is strictly bilateral-symmetrical, i.e., we 

 can divide their bodies longitudinally into two equal parts. This 

 applies to fishes as well as to Man. If we know one side of the 

 body we shall be able to reconstruct the other. In particular, 

 the extremities and the most important sense organs eyes and 

 ears are distinguished by their uniform distribution upon the 

 right and left half of the body. 



A remarkable exception is furnished by the flat-fishes plaice, 

 flounders, turbot, and soles which form one of our staple foods 

 and are therefore universally known. Their structure seems to 

 controvert all our conceptions concerning the construction of 

 the vertebrates, but this deviation from the normal state is due 

 to an unusually strong lateral flattening of the whole body. As 

 a result of this process the distance between the dorsal and 

 ventral surface becomes enlarged while that between the sides 

 is shortened. Because the formation of the right and left half 

 is widely different in appearance, and because the flat-fishes, 

 moreover, swim laterally due to the strong lateral compression 

 of the body most observers would unhesitatingly describe one 

 side as the back and the other as the abdomen. 



The underside of the flat-fishes, corresponding to the right or 



