

174 VERTEBRATE ZOOLOGY 



suit of their somewhat anomalous lives, involving the necessity of an 

 amphibious environment. They are forced to occupy a narrow strip 

 of territory between the waters and the dry land, a prey to the domi- 

 nant denizens of the waters (fishes) on the one hand and to the vari- | 

 ous more vigorous enemies on the land (reptiles, birds, and mammals) ! 

 on the other. If hard pressed in one environment the amphibian 

 may seek the other; and this has saved him from complete extinction. 



The Amphibia to-day are represented largely by a single highly spe- 

 cialized order, the Anura^ (frogs and toads), that have undergone 

 within comparatively recent times a wonderfully elaborate adaptive 

 radiation into a great variety of habitat complexes. But for the 

 Anura the modern Amphibia would be largely unknown, for the sala- 

 manders, newts, and csecilians are furtive, inconspicuous forms that 

 have sought safety irf the liidden nooks and crannies of the world 

 environment and persist through their extremely retiring habits. 



At one time, however, the Amphibia occupied a comparatively 

 honorable place in nature. They reached in some cases almost giant 

 size and evidently were active and predaceous creatures. Their wane 

 began with the rapid rise of the Reptilia, which, as a group, became 

 much more completely adjusted to land life than did the Amphibia. 



THE ORIGIN or THE AMPHIBIA 



It is now generally admitted that the Amphibia arose as a lateral 

 branch from a very early group of "lobe-fin" ganoids (Crossopterygii). 

 The time of emergence of the first Amphibia appears to have been 

 about Middle Devonian, the period when the fishes gained their 

 earliest pronounced ascendancy and when all of the available habi- 

 tat complexes were occupied. The earliest trace of amphibian life 

 is a single footprint (Fig. 101) of a three-toed species (Thinopus an- 

 tiquus) found in the Upper Devonian shales of Pennsylvania. * This 

 foot though primitive was a true foot; not a fin. It is therefore prob- 

 able that there were many transitional stages from the fin to the foot 

 which are beyond our ken, and that the transition occupied at least 

 thousands of years. The skeletal structure of the lobe-fin ganoid 

 paired fins, especially that of the pectorals, is quite hand-like in ar- 

 rangement ; so that a dropping off of the f ririge-like fin portion would 

 leave a structure quite like a hand with three or more fingers (Fig. 102, 

 C, D, E). The dropping of the fin-fringe may have happened quite 

 suddenly in the process of evolution of some group possibly by a 



