EVALUATION OF CAUSES IN RISE OF AMPHIBIANS 415 



native environment have been a cause competent to account for the 

 passage to terrestrial life ? The answer seems clear that it was not a true 

 cause. 



Such a pressure of environment has been always present, yet on lands 

 free from enemies has not led to renewed adaptations among fishes. 

 Those few which crawl on the land do not do so to escape their enemies. 

 The carnivorous enemies of a group of animals can not cause extinction 

 without the aid of other causes, unless those carnivores live also on other 

 kinds of food; for when the numbers of the food animals become re- 

 duced the species ceases to form a sufficient food for the carnivorous foes. 

 A balance is therefore struck between the large number of herbivores and 

 a smaller number of their carnivorous foes. Furthermore, in the case of 

 the amphibians, the young stages were still spent in the water. Where 

 safety of environment is sought, it is especially in the breeding season 

 that it becomes necessary. An even more conclusive argument against 

 such an accusation of weakness concerning the first breathers of the air 

 is found in the formidable teeth and head armor with which the first of 

 amphibians were endowed. As soon as they appear under conditions 

 where their bones could be preserved, they are clearly carnivores and the 

 dominant creatures of their habitat. 



FOOD OF THE LANDS, AN INADEQUATE CAUSE 



The hypothesis that there was a pressure from enemies in the rear 

 which forced the amphibians from the waters appears contrary to the 

 evidence; but as the next step in the analysis of causes must be consid- 

 ered, the possible efficiency of unused foods on the lands as a bait or lure 

 to draw the fish from the waters. The lands have supplied food pre- 

 sumably since at least early in the Paleozoic. At first low, cellular cryp- 

 togamous vegetation constituted the only organic covering, then vascular 

 cryptogams, arachnids, and primitive insects appeared. The variety and 

 quality of plant and insect foods have increased throughout the geologic 

 ages. The temptation to avail themselves of such foods is ever before 

 those marine animals which live in the tidal zone. To what degree have 

 the animals followed up that temptation? A host of worms, mollusks, 

 and crustaceans dwell there, yet when the receding tide leaves them 

 stranded the worms and crustaceans burrow into mud or under stones to 

 await the return of the waters. Many mollusks close their shells and cer- 

 tain small fishes burrow in sand or moss or in the chinks between stones. 



Even on isolated lands where terrestrial enemies are absent there is 

 shown but little tendency for marine animals to pass actively beyond the 

 reach of the waters. The land crabs and the jumping-fish (Periophthal- 

 XXX — Boll. Gbol. Soc. Am., Vol. 27, 1915 



