150 THE GENESIS OF SPECIES. [CHAP 



over the greater part of the globe. Yet no single transi- 

 tional form has yet been met with in spite of the multitu- 

 dinous individuals preserved. Again, with regard to their 

 modern representatives the Cetacea, one or two aberrant 

 forms alone have been found, but no series of transitional 

 ones indicating minutely the line of descent. This group, 

 the whales, is a very marked one ; and it is curious, on 

 Darwinian principles, that so few instances tending to in- 

 dicate its mode of origin should have presented themselves. 

 Here, as in the bats, we might surely expect that some 

 relics of unquestionably incipient stages of its development 

 would have been left. 



SKELETON OF A PLESIOSAURUS. 



The singular order Chelonia, including the tortoises, 

 turtles, and terrapins (or fresh- water tortoises), is another 

 instance of an extreme form without any, as yet known, 

 transitional stages. Another group may be finally men- 

 tioned, viz. the frogs and toads (anourous Batrachians), 

 of which we have at present no relic of any kind linking 

 them on to the Eft group on the one hand, or to Eeptiles 

 on the other. 



The only instance in which an approach towards a series 

 of nearly related forms has been obtained is that of the 

 existing horse, its predecessor Hipparion and other extinct 

 allies. But even here we have no proof whatever of modi- 



