2 LARGE SIZE OF MAMMALIA CHAP. 



as a biting organ. The term highest, however, includes increased 

 complexity as well as simplification, the two series of modifica- 

 tions being interwoven to form a more efficient organism. It 

 cannot be doubted that the increased complexity of the brain of 

 mammals raises them in the scale, as does also the complex and 

 delicately adjusted series of bonelets which form the organ for 

 the transmission of sound to the internal ear. The separation of 

 the cavity containing the lungs, and the investment of the parti- 

 tion so formed with muscular fibres, renders the action of the 

 lungs more effective; and there are other instances among the 

 Mammalia of greater complexity of the various parts and organs 

 of the body when compared with lower forms, which help to 

 justify the term "highest" generally applied to these creatures. 



Complexity and finish of structure are often accompanied by 

 large size ; and the Mammalia are, on the whole, larger than any 

 other Vertebrates, and also contain the most colossal species. The 

 huge Dinosaurs of the Mesozoic epoch, though among the largest 

 of animals, are exceeded by the Whales ; and the latter group 

 includes the mightiest creature that exists or has ever existed, 

 the eighty-five-feet-long Sibbald's Rorqual. Confining ourselves 

 rigidly to facts, and avoiding all theorising on the possible 

 relation between complexity and nicety of build and the capacity 

 for increase in bulk, it is plain from the history of more than 

 one group of mammals that increase in bulk accompanies specialis- 

 ation of structure. The huge Dinocerata when compared with 

 the ancestral Pantolamlda teach us this, as do many similar 

 examples. Within the mammalian group, as in the case of other 

 Vertebrates, difference of size has a certain rough correspondence 

 with difference of habitat. The Whales not only contain the 

 largest of animals, but their average size is great ; so too with 

 the equally aquatic Sirenia and very aquatic Pinnipedia. Here 

 the support offered by the water and the consequent decreased 

 need for muscular power to neutralise the effects of gravity 

 permit of an increase in bulk. Purely terrestrial animals come 

 next ; and finally arboreal, and, still more, " flying " mammals 

 are of small size, since the maintenance of the position when 

 moving and feeding needs enormous muscular effort. 



The Mammals are more easily to be separated from the 

 Vertebrates lying lower in the series than any of the latter are 

 from each other in ascending order. A large number of char- 



