AND ITS INHABITANTS 123 



seventy in the latter part of that period. This meant, as in 

 Australia today, the reduction of rivers and other bodies of 

 fresh water and the entailed concentration of their fauna, 

 which is borne out by the mode of occurrence of the Lower 

 Devonian (Old Red Sandstone) fishes innumerable speci- 

 mens in very restricted areas. Add to this the diminution of 

 aeration of these waters and it will be seen that a high premium 

 would be placed upon powers of air breathing or of aestivation. 

 Still further desiccation would necessitate some sort of activity 

 during the increasingly long droughts, for the periods of torpor 

 would otherwise bear too great a ratio to the creature's life 

 span. Thus a premium would be placed upon ability to crawl 

 ashore and maintain an active life, while the less fit would sleep 

 the sleep that knows no waking, to their racial extinction. 



Aridity, therefore, would place a premium, first upon lung 

 breathing, the first recorded lung-breathing fish appearing in 

 the Lower Devonian, although they must have existed in the 

 Silurian ; and later upon emergence, the entire process up to the 

 perfection of the terrestrial limb covering more than the whole 

 Devonian period, as the earliest known footprint of a terres- 

 trial vertebrate is found in Upper Devonian rocks. 



Evolution of the terrestrial foot. The question of the evo- 

 lution of the pentadactyl hand and foot, which is the vertebrate 

 standard, from the ancestral fish fin is not fully solved, but 

 much light is thrown upon it by the above-mentioned footprint, 

 known to science as Thinopus antiquus and preserved in the 

 Peabtfdy Museum at Yale. The slab of sandstone bears a 

 single track (PL III), that of a right foot having two well- 

 developed digits with distinct phalangeal impressions. On the 

 outer side of the second digit is a budding third, while lower 

 down on the side of the foot may be seen the rudiment of a 

 fourth. Rabl 5 has shown that the developing foot of the 



5 Rabl, C., Gedanken und Studien iiber den Ursprung der Extremitaten. 

 Zeits. fur wiss. Zoologie, vol. 70, 1901, pp. 474-558. 



