114 THE INDUCTIONS OF BIOLOGY. 



the animal kingdom, some case wtere classes otherwise 

 allied, are contrasted in their locomotive activities. Let ua 

 compare birds on the one hand, with reptiles and mammals 

 3n the other. It is an accepted doctrine that birds are 

 organized on a type closely allied to the reptilian type, but 

 superior to it ; and though in many respects the organization 

 of birds is inferior to that of mammals, yet in other respects, 

 as in the greater heterogeneity and integration of the; skeleton, 

 the more complex development of the respiratory system, 

 and the higher temperature of the blood, it may be held 

 that birds stand above mammals. Hence were growth de- 

 pendent only on organization, we might infer that the limit 

 of growth among birds should not be much short of that 

 among mammals ; and that the bird-type should admit of a 

 larger growth than the reptile-type. Again, w^e see no mani- 

 fest disadvantages under which birds labour in obtaining 

 food, but from which reptiles and mammals are free. On the 

 contrary, birds are able to get at food that is fixed beyond 

 the reach of reptiles and mammals ; and can catch food that 

 is too swift of movement to be ordinarily caught by reptiles 

 aiid mammals. Nevertheless, the limit of growth in birds, 

 falls far below that reached by reptiles and mammals. With 

 what other contrast between these classes, is this contrast 

 connected ? May we not suspect that it is connected with 

 the contrast between their amounts of locomotive exertion ? 

 Whereas mammals (excepting bats, which are small), are 

 during all their movements supported by solid surfaces or 

 dense liquids ; and whereas reptiles (excepting the ancient 

 pterodactyles, which were not very large), are similarly re- 

 stricted in their spheres of movement ; the majority of birds 

 move more or less habitually through a rare medium, in which 

 they cannot support themselves without relatively great 

 efforts. The conclusion that there exists this inverse 



ratio between growth and expenditure of force, is enforced 

 by the significant fact, that those members of the class Aves, 

 as the Dinornis and Epiornis, which approached in size to 



