146 THE GENESIS OF SPECIES. [CHAP. 



or three can be selected as examples. Thus those remark- 

 able fossil reptiles, the Ichthyosauria and Plesiosauria, ex- 



THE AECHEOPTEKYX (OF THE OOLITE STRATA). 



tended, through the secondary period, probably over the 

 greater part of the globe. Yet no single transitional form 

 has yet been met with in spite of the multitudinous indi- 

 viduals preserved. Again, with their modern representa- 



SKELETON OF AN ICHTHYOSAURUS. 



tives, 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, 



