ICHTHYOSAURUS. 145 



ships by their sails, and to steam-boats by their paddles, is 

 similarly placed. The great organ of motion in fishes, the 

 tail, is indeed posteriorly placed, but this by its mode of ac- 

 tion generates a vis a tergo, which impels the animal straight 

 forwards, and does not therefore operate under the same 

 conditions with organs laterally applied." G. T. V. 5, p. 

 579. 



I shall conclude this detailed review of the pecuUarities 

 of one of the most curious, as well as the most ancient, 

 among the many genera of extinct reptiles presented to us 

 by Geology, with a few remarks on the final causes of 

 those deviations from the normal structure of its proper 

 type, the Lizard ; under which the Ichthyosaurus combines 

 in itself the additional characters of the fish, the Whale, and 

 Ornithorhynchus. As the form of vertebra by which it is 

 associated with the class of fishes, seems to have been in- 

 troduced for the purpose of giving rapid motion in the water 

 to a Lizard inhabiting the element of fishes ; so the farther 

 adoption of a structure in the legs, resembling the paddles of 

 a Whale, was superadded in order to convert these extremi- 

 ties into powerful fins. The still farther addition of a fur- 

 cula and clavicles, like those of the Ornithorhynchus, oflfers 

 a third and not less striking example of selection of contri- 

 vances, to enable animals of one class to live in the element 

 of another class. 



If the laws of co-existence are less rigidly maintained in 

 the Ichthyosaurus, than in other extinct creatures which we 

 discover amid the wreck of former creations, still these de- 

 viations are so far from being fortuitous, or evidencing im- 

 perfection, that they present examples of perfect appointment 

 and judicious choice, pervading and regulating even the 

 most apparently anomalous aberrations* 



Having the vertebra? of a fish, as instruments of rapid 

 progression; and the paddles of a Whale, and sternum of 

 an Ornithorhynchus, as instruments of elevation and depres- 

 sion ; the reptile Ichthyosaurus united in itself a combination 



VOL. I. — 13 



