120 THE GENESIS OF SPECIES. [CHAP. 



the latest secondary rocks, so that their development could 

 not have been so very slow, unless geological time is (al- 

 though we shall presently see there are grounds to believe 

 it is not) practically infinite. It is quite true that it is, in 

 general, very unsafe to infer the absence of any animal 

 forms during a certain geological period, because no re- 

 mains of them have as yet been found in the strata then de- 

 posited : but in the case of the Cetacea it is safe to do so ; 

 for, as Sir Charles Lyell remarks, 26 they are animals, the re- 

 mains of which are singularly likely to have been preserved 

 had they existed, in the same way that the remains were 



SKELETON OF A PLE8IOSATJRU8. 



preserved of the Ichthyosauri and Plesiosauri, which ap- 

 pear to have represented the Cetacea during the secondary 

 geological period. 



As another example, let us take the origin of wings, 

 such as exist in birds. Here we find an arm, the bones of 

 the hand of which are atrophied and reduced in number, as 

 compared with those of most other Vertebrates. Now, if 

 the wing arose from a terrestrial or subaerial organ, this 

 abortion of the bones could hardly have been serviceable 

 hardly have preserved individuals in the struggle for life. 

 If it arose from an aquatic organ, like the wing of the pen- 

 guin, we have then a singular divergence from the ordinary 



25 Principles of Geology," last edition, vol. i., p. 163. 



