THE EXTERNAL FORM OF WHALES 3 



interesting to inquire into the reasons for their 

 excess of bulk over the animal world in general. 

 There are various causes which seem to contribute 

 to the acquisition of a mighty frame. In the first 

 place, the medium in which the animal lives must 

 have something to do with it. Aquatic creatures 

 have naturally less difficulty in sustaining a colossal 

 bulk than have animals which live in a less dense 

 medium. We find, in fact, a distinct relation between 

 size and habitat. " The blue shark, Carcharias" 

 remarked the late Professor Milnes Marshall, 

 "attains a length of 25 feet; specimens of Carcharo- 

 don have been measured over 40 feet in length ; 

 while of the genus Rhinodon examples of 50, 60, 

 or even 70 feet in length have been described." 

 Purely volant animals, bats, birds, and pterodactyles, 

 have far greater difficulties in sustaining themselves 



o o 



in the air ; hence these classes of animals are 

 relatively small. We may believe in /Epyornis, but 

 we cannot accept a flying Roc. The middle position 

 is occupied by mammals, which require more muscular 

 effort to stand or crawl than aquatic creatures, but 

 not nearly so much as aerial. We find that their 

 size is in correspondence. The Mastodon and the 

 great ground sloths were larger than any pterodactyle 

 or bird, but not so large as whales. The Dinosaurs 

 are thought by some to have been at least partially 

 aquatic, to have frequented at least marshes and 

 estuaries. But, even if they were purely terrestrial, 

 they do not acquire absolutely the same colossal 

 dimensions as do some whales. 



