PALAEONTOLOGY AND GEOLOGY 



and life because of lack of food. But we have no relics 

 of amphibians in a transitional state in the Subcar- 

 boniferous age. "Relics of amphibians appear in abun- 

 dance only in later Coal Measures. They were already 

 differentiated into five sub-orders."^* The whole gap 

 of this enormous change has to be filled by this single 

 discovery in the Subcarboniferous period : "The most 

 interesting suggestion of advance in land life is found 

 in the footprints of a supposed amphibian named 

 Peleosauropus primaevus^ described by Lea from the 

 Mauch Chunk shale near Pottsville, Pennsylvania. 

 There are six double imprints, in which the track of 

 the hind foot partially covers that of the front foot. 

 The trail of a tail an inch wide accompanies the foot- 

 prints. The slab on which they are impressed is ripple- 

 marked and pitted by rain-drops, implying a freshly 

 emerged mud-flat again covered before the impres- 

 sions were lost."^" The positive evidence of so mo- 

 mentous a change of structure to be derived from six 

 footprints seems a slender one on which to base the 

 continuity of evolution. 



We may now pass to the Jurassic period. Just as 

 land locomotion appeared fully developed so also 

 does the presence of feathered birds, and their ances- 

 try is admitted to be a puzzle. "The ancestors of the 

 pterosaurs [reptiles with batlike wings] and the birds 

 may doubtless have been closely allied far back 



^s Chamberlain and Salisbury, vol. II, p. 607. 

 19 Ibid., vol. II, p. 537. 



c 1553 



