142 THE INDUCTIONS OF BIOLOGY. 



allied, arc contrasted in their locomotive activities. Let us 

 compare birds on the one hand, with reptiles and mammals 

 on 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 some 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, we 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 

 and 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 (par- 

 tially though not wholly) with the contrast between their 

 amounts of locomotive exertion ? Whereas mammals (except- 

 ing 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 restricted in their spheres of move- 

 ment; the majority of birds move more or less habitually 

 through a rare medium, in which they cannot support them- 

 selves without relatively great efforts. And this general fact 

 may be joined with the special fact, that those members of 

 the class Aves, as the Dinornis and Epiornis, which ap- 

 proached in size to the larger Mammalia and Reptilia, were 

 creatures incapable of flight creatures which did not expend 



