SPECIAL PRECAUTIONARY HABITS 329 



(Notoryctes) of Australian deserts, belonging to the still lower 

 order of Marsupialia. Such Reptiles as the small degenerate 

 Burrowing- Snakes (Typhlopida) and certain Snake-like Lizards 

 (Ampkisbanida) are further examples, and still another instance 

 is afforded by the curious tropical Caecilians, constituting an order 

 in >Amphibia which present a superficial resemblance to Serpents. 

 It may be noted here, too, that a rich fauna inhabits certain caves. 



This section may appropriately be concluded by a few remarks 

 on protective measures as to place of feeding which have been 

 adopted by aquatic forms. 



CERTAIN MARINE ANIMALS WHICH HAVE GAINED PROTECTION 

 BY MIGRATING TO NEW FEEDING-GROUNDS. There is good reason 

 for believing that the sea is the original home of life, and from 

 very ancient times its shallow waters up to high-tide mark have 

 been the scene of very keen competition for food. By migration 

 from shallow water in different directions certain forms tapped 

 fresh sources of food-supply, and, for a time at any rate, succeeded 

 in reducing the toll levied upon them by predaceous forms. It 

 is extremely probable that many Land-Animals originally took 

 origin from the fauna existing between tide-marks, the conditions 

 there being such as to afford a preparatory training for terrestrial 

 existence pure and simple. Backboned Land-Animals most likely 

 sprang from freshwater forms (see next paragraph), and these 

 again claim marine ancestry. The land, once peopled, soon pre- 

 sented a struggle for existence as keen as that obtaining in the 

 sea, and in the ways already indicated the pressure was partially 

 relieved. Another particularly interesting case here deserves 

 mention, i.e. that of various forms which, after becoming thorough- 

 going land animals, returned to the original home of life, and 

 underwent a second series of specializations fitting them for a 

 new sort of marine existence. In this way two of the orders 

 of Mammals have been evolved: Cetacea, including Whales, 

 Porpoises, &c. ; and Sirenia, comprising Manatees and Dugongs. 

 Among the Carnivora, too, we have the Pinnipedia (Walruses, 

 Sea- Lions, and Seals), which live more in the sea than out of 

 it. A similar policy has been pursued by members of some 

 other classes of land Vertebrates. Penguins, for example, have 

 given up flight, and spend a large part of their lives in the sea, 

 using their modified wings as paddles. Several groups of extinct 

 Reptiles appear to have lived entirely in the sea; why they died 



