294 OUTLINES OF EVOLUTIONARY BIOLOGY 



breathing by means of gills (Fig. 121). The newts, on the other 

 hand, though developing lungs, retain the fish-like form and the 

 aquatic habit throughout life. 



In one respect the imperfection of the geological record as 

 regards the origin of the Amphibia from fishes is especially to 

 be regretted. One of the chief distinctions between the two 

 groups lies in the fact that whereas the Amphibia always 

 have pentadactyl limbs (except when these have been lost 

 by degeneration), the fishes have only fins, which are not 

 pentadactyl. The conversion of the fin into a pentadactyl limb 

 was no doubt an adaptation to the terrestrial mode of life, but 

 neither comparative anatomy nor embryology nor yet the evidence 

 of the geological record have so far enabled us to discover how the 

 transformation of the one type of limb into the other actually took 

 place. We can only hope that light may yet be thrown upon this 

 difficult problem by the discovery of the fossil remains of inter- 

 mediate forms. 



The reptiles and the amphibians are closely related to one 

 another, but existing reptiles are readily distinguished from 

 existing amphibians in a variety of ways. For example, they 

 produce much larger eggs, within which the young are nourished 

 up to a very advanced stage of development by means of the yolk, 

 while the embryo is provided with the special protective and 

 respiratory total membranes known as amnion and allantois, 

 which are not found in amphibians. A reptile, again, never 

 develops gills at any stage of its existence, while an amphibian 

 not only has gills in the larval but sometimes also in the adult 

 state. 



Fossil remains naturally afford no clue as to when the develop- 

 ment of the total membranes and the suppression of the larval 

 gills took place, and we are thrown back entirely upon skeletal 

 peculiarities as a means of distinguishing between extinct 

 members of the two groups. From one point of view this is 

 unsatisfactory, but the very difficulty which we experience in 

 drawing a hard and fast line between the fossil reptiles and 

 amphibians only serves to emphasize the fact that the one group 

 has been derived from the other. 



One of the chief osteological peculiarities which distinguish 

 existing reptiles from amphibians is, as Dr. Smith Woodward 

 observes, " the degeneration of the parasphenoid bone and its 

 functional replacement in the basi-cranial axis by the pterygoids. 



