122 EVOLUTION OF THE EARTH 



mud in the river bottom and activates 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, Polypterus 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 living 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 



