IV.] MINUTE MODIFICATIONS. 123 



Bats were, by the earliest observers, naturally supposed 

 to have a close relationship to birds, and cetaceans to fishes. 

 It is almost superfluous to observe that all now agree that 

 these mammals make not even an approach to either one 

 or other of the two inferior classes. 



In the same way it has been recently supposed that 

 those extinct flying saurians, the pterodactyls, had an 

 affinity with birds more marked than any other known ani- 

 mals. Now, Iwwever, as has been said earlier, it is con- 

 tended that not only had they no such close affinity, but 

 that other extinct reptiles had a far closer one. 



The amphibia (i. e., frogs, toads, and efts) were long 

 considered (and are so still by some) to be reptiles, show- 

 ing an affinity to fishes. It now appears that they form 

 with the latter one great group the ichfchyopsida of Prof. 

 Huxley which differs widely from reptiles ; while its two 

 component classes (fishes and amphibians) are difficult to 

 separate from each other in a thoroughly satisfactory man- 

 ner. 



If we admit the hypothesis of gradual and minute mod- 

 ification, the succession of organisms on this planet must 

 have been a progress from the more general to the more 

 special, and no doubt this has been the case in the majority 

 of instances. Yet it cannot be denied that some of the 

 most recently-formed fossils show a structure singularly 

 more generalized than any exhibited by older forms ; while 

 others are more specialized than are any allied creatures 

 of the existing creation. 



A notable example of the former circumstance is offered 

 by macrauchenia a hoofed animal, which was at first sup- 

 posed to be a kind of great llama (whence its name) the 

 llama being a ruminant, which, like all the rest, has two 

 toes to each foot. Now hoofed animals are divisible into 

 two very distinct series, according as the number of func- 

 tional toes on each hind-foot is odd or even. And many 



