VARIATION AND ADAPTATION 225 



An interesting adaptation to the habit of digging occurs in 

 the spade-footed toads of Europe, Pelobates and Scaphiopus. 

 The inner tarsal (ankle) tubercle which occurs in Anura gener- 

 ally is developed into a curved oblique ridge covered with a 

 hard horny sheath with a sharp edge ; this organ is used as a 

 spade or shovel with which the animal digs its way into soft 

 soil, especially into sandy ground. The loose sand falls on the 

 toad as it shovels away that which is underneath and so it sinks 

 backwards into the ground quite rapidly and remains buried in 

 the daytime, emerging at night in search of food. This is one 

 of the cases which to a Lamarckian seems inexplicable except 

 on the theory of the inheritance of the effects of external stim- 

 ulations. We can scarcely doubt that if the feet were used 

 for digging as they actually are used, the friction would cause 

 a thickening and cornifkation of the epidermis at the part most 

 affected, and if such effects were inherited the organ as it exists 

 would be produced. The opposing view is that although in 

 the individual the epidermis would be thickened and hardened, 

 yet the evolution of the structure was due to the selection of 

 spontaneous variations which were independent of the friction 

 due to the act of digging. 



The most surprising fact concerning the respiratory organs 

 of Amphibia is the absence of lungs as well as gills in certain 

 species of Urodela. These species all belong to the family 

 Salamandridae, in which the gill and gill-clefts are normally lost 

 in the adult condition. The lungs may be entirely absent or 

 reduced to a vestigial condition of no functional value. All the 

 Desmognathinse and Plethodontinae, so far as known, are lung- 

 less and also Amblystoma opacum and Salamandrina perspicil- 

 lata. With the disappearance of the lungs are correlated certain 

 changes in the circulatory organs : the pulmonary veins are 

 naturally absent and the left auricle is much reduced in size 

 and not completely separated from the right, so that the heart 

 has practically only two chambers, one auricle and one ventricle, 

 as in a fish. In all Amphibia the skin is an important organ 

 of respiration and we know that in the hibernating condition 

 the animals depend on the skin alone for the little oxygen 

 which is required. According to one investigator the skin is 

 the respiratory organ in the lung-less salamanders, but others 

 maintain that the action of the skin is unimportant and that the 



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