AND ITS INHABITANTS 125 



embryonic amphibian, Triton (Fig. 20), passes through a 

 series of stages of which one corresponds precisely to the 

 degree of development of the Devonian track, which, from 

 its size, is probably that of an adult although yet in the 

 adolescence of its race. Comparative anatomy corroborates 

 this belief and completes the tale of evidence (Fig. 21), in 

 that the principal axes of the foot, as shown by the distribution 

 of nerves and blood-vessels, lie in the first and second digits, 

 the lesser axes of digits III, IV, and V arising as lateral 

 branches from that of digit II. 



With the opening of Mississippian time came increased 

 moisture and in the succeeding widely extended swampy forests 

 of the coal period, amphibia throve mightily and developed 

 into the many sorts of so-called Stegocephalia or armored 

 forms. That they still returned to their ancestral waters to 

 bring forth their young, and that the latter bore gills upon the 

 neck for aquatic respiration is evidenced by the actual traces 

 of such structures in many fossil forms. 



FIG. 22. Restoration of the Permian stegocephalian, Cacops 

 aspidephorus. After Williston, from the Pirsson-Schuchert 

 "Text-book of Geology," published by John Wiley & 

 Sons, Inc. 



Origin of reptiles. During the latter half of the Mississip- 

 pian, however, came a diastrophic movement with a wave of 

 aridity, making the return to the natal waters increasingly 

 difficult until many forms were forced to abandon it forever, 

 and the reptiles came into being. There are today well- 

 recognized criteria whereby a reptile may be distinguished 



