The Geological Lesson. 99 



first into legs and legs into flippers. So that the leg stage of the pro- 

 cess would gradually be eliminated and more or less of a cutoff be 

 made, and while it is an advantageous modification or adaptation, it is 

 nevertheless a retrogression. 



In applying evolution to the details of animal advance as we find it 

 in geology, it is necessary to keep this consideration in view ; because 

 frequently the first species we happen to find of a new type or family is 

 not the lowest or least developed of that family. Thus no Ornithorhyn- 

 chus or Echidna has been found fossil, although they belong to the low- 

 est order of the mammalia. But this animal may be descended from an 

 ancestor common to himself and the* Marsupials, and while he has be- 

 come adapted to an inferior mode of life, the Marsupial has diverged in 

 the other direction. They both have Marsupial bones which are prob- 

 ably the remains of a posterior sternum possessed by the common an- 

 cestor, and which have been* turned to use in the kangaroo, &c. , in sup- 

 porting the marsupium or pouch. 



In like manner the Edentates are first found in the Miocene, while 

 both herbivores and carnivores, superior to them in organic structure, 

 appeared in the epoch before. But the Edentates do have teeth at first 

 which are good enameled mammal teeth ; but they lose them and get 

 others of an inferior development in their place. They are thus shown 

 to be degenerated from the higher mammals and so ought to come later. 

 There are no doubt many other such cases in which the later inferior 

 species are descended from superior by retrogressive adaptation. 



Again, Snakes do not appear till the Lower Tertiary, while their 

 superiors, the Turtles and Lizzards, occur in the Triassic. But Snakes 

 may be a later off-shoot of the Amphibian Salamanders, as the Saurians 

 were an earlier, or they too may be degenerate amphibians. The Sala- 

 manders continued to be Salamanders after they diverged from the 

 Reptiles and it is not improbable that when land appeared suitable for a 

 new form of reptile, a family of Salamanders might become modified by 

 it into the snake form. 



Many of the Snake tribes have rudimentary limbs, 'which in some 

 cases appear outside of the skin as small hooks, in other cases do not 

 cut through the skin, and are only insignificant rudiments or vestiges. 

 The lungs in most species are more or less aborted ; that is, one of 

 them is reduced while the other is full sized. These are marks of retro- 

 gressive adaptation and show the Snake to be a modified form of 

 Amphibian reduced in respect to limbs and lungs. On the other hand, 

 their reproducti\e apparatus and process are very superior to the 

 Amphibian, since they reproduce by eggs internally impregnated, and in 

 the case of the vipers, the eggs are internally hatched and the young 

 born alive ; while the Amphibian eggs are first laid and impregnated 



