30 LIFE ON THE EARTH. 



malia are separated from all other animals by their 

 mode of rearing their young. 



Among Mammalia quadrupedal motion on land 

 appears to be the reigning idea, which determines the 

 complete type ; but the Cetacea, destined for aquatic 

 life, are bipedal, and have their two legs altered to 

 perform the work of pectoral fins, and the tail ex- 

 panded to a propulsive instrument. Thus ideas are 

 expressed which seem borrowed from fishes, to which 

 in general form suited for easy motion in water the 

 Cetaceans also correspond. So in early geological 

 times we find Ichthyosaurus assuming that sort of 

 conformity to fish-structure and form, which belongs 

 to what Mac Leay calls analogy, while the real affinity 

 of the whale is to ordinary Mammalia, and the real 

 affinity of the Ichthyosaurus is to ordinary Reptilia. 



In a different manner the quadrupedal mammal 

 is modified for flight in the air. The Bat takes up the 

 extended interdigital membrane, and the sternal keel, 

 by analogy with birdsas in older periods of the 

 world, the Pterodactyle spread its wings and worked 

 its pectoral muscles under the same peculiarities 1 . 

 Instances of this kind might be greatly multiplied, 

 but these may be sufficient to shew that each great 

 type or subtype of structure admits of variations of 



1 See the evidence of the sternal keel of Pterodactylus in the 

 fine collection of Green Sand Fossils in the Cambridge Museum. 



