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 



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



I 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 stegocephallan, 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- 



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



jaridity, making the return to the natal waters increasingly 



|iifficult until many forms were forced to abandon it forever, 



!ind the reptiles came into being. There are today well- 



ccognized criteria whereby a reptile may be distinguished 



