122 EVOLUTION OF THE EARTH 



mud in the river bottom and aestivates therein; and the South 

 American lung-fish, Lepidosiren, of the Amazon River and its 

 affluents, which lives near the margin of the water, using its 

 lung almost with the regularity of a mammal, and also forming 

 a burrow for its habitation during the dry season. Of these 

 dipnoans the latter two belong to a group of which we have 

 no fossil record; Neoceratodus, on the other hand, is a relic 

 of what was formerly a large and widespread group. Yet 

 another order of fishes in which the air-bladder has a respira- 

 tory value is the Crossopterygii, or fringe-finned ganoids, again 

 an important and numerous group in the geologic past but now 



Fig. 19. — African fringe-finned ganoid, Folypteriu delhezi. After Jordan, from 

 Lull's **Organic Evolution," published by the Macmillan Company. 



represented by but two genera, Polypterus (Fig. 19) and 

 Erpetoichthys, both tropical African in distribution. While 

 these Hving genera have not so effective a respiratory device 

 as the dipnoans, nevertheless they present fewer anatomical 

 difficulties to stand in the way of relationship with the am- 

 phibia. Without rehearsing the technical arguments, it may 

 suffice to say that the generally accepted view is that the terres- 

 trial vertebrates were derived either from ancient crossop- 

 terygians or from a group ancestral to both them and dipnoans. 

 The geologic cause which lies back of the emergence is 

 apparent. Diastrophic movement during the Silurian period 

 (see Fig. 14) 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 



