150 EVOLUTION AND GENETICS 



larvae are said to resemble those of Amphibia. In short, the 

 Crossopterygii have a number of the characters of the hypo- 

 thetical ancestor of terrestrial vertebrates. 



The Period of Transition. In the geological record a very 

 significant fact is the abundance of these fishes in the Devonian, 

 the period which also saw the rise of the Amphibia. 



During this time, the paleontologists tell us, climatic conditions 

 were such as to force upon animals the change from aquatic to 

 terrestrial life. As Lull expresses it, "Diastrophic movement 

 during the Silurian period initiated a widespread aridity which 

 culminated in the latter part of the period, continued with varying 

 intensity into and through Devonian time, and rose again to greater 

 severity in the latter part of that period. This meant, as in Aus- 

 tralia 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 specimens 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 dessication 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 Hfe 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." 



The Amphibia. Once able to exist on land there would be 

 abundant reason for animals to continue their development toward 

 greater fitness for the new mode of Hfe. In the waters would be 

 concentrated all of their ancient enemies; on land would be freedom 

 up to the limits imposed by their own structure. The story of 

 vertebrate succession, already touched upon many times, shows 

 by what means this has led up to the maximum development of 

 the class. The geological record shows how many species, once 

 successful, have fallen before unfavorable conditions because they 

 were unable to meet the requirements of a changing environment. 



The development of the Amphibia as terrestrial organisms 

 probably l^egan in the Middle Devonian period and extended 

 through the Carboniferous when the drying of the earth's surface 

 produced vast swampy areas. Restriction of bodies of water and 



