246 THE POPULAR SCIENCE MONTHLY. 



examples of swimming and running birds without fliglit, and of 

 marine and aerial mammals, have already been given. In these 

 cases, the fundamental structures become in part worn and mu- 

 tilated, and some of them entirely lost. Some of the running 

 birds have lost a large part of the bones of the wing, and the 

 whales have lost the hinder limbs. Such mutilation and loss of 

 parts is proof that the animal which has suffered them has de- 

 parted from the environment of its ancestors. 



But though deflection and antagonism of structures is possi- 

 ble and frequent, it is necessarily not usual ; later modifications 

 of structures are ordinarily in harmony with more fundamental 

 ones, and later conditions of environment with primary ones. 

 Progress is ordinarily easiest in a straight line. Most fishes live 

 in the water and swim, though they become variously modified 

 for the various secondary conditions found in this medium. 

 Most birds fly, though they are subjected to endless modifica- 

 tions which are in harmony with flight. The mass of shot show 

 the spot aimed at, and not the scattering pellets. When later 

 modifications are in agreement with primary ones, the primary 

 structures remain in full perfection and use. 



The facts of environment bearing upon life are so various 

 and so heterogeneous that they allow of but little classification. 

 Those conditions which have existed pretty generally over the 

 earth, and with little or no change since the creation of life, 

 have had the most j^rofound modifying influence. Among these 

 are the different mediums respired, air and water, and the differ- 

 ent horizons or locations requiring peculiar organs of locomo- 

 tion, deep and shallow water, earth, rocks, and trees, and air. 

 The divisions of types which are usually dignified by systema- 

 tists with the title of classes have their reason for existence in 

 conditions of this kind. The five recognized classes of verte- 

 brates — fishes, amphibians, reptiles, birds, and mammals — are 

 simply modifications of type for life in water, in shallows, on 

 land, on trees and rocks, and in the air. 



But these conditions are not confined to this first influence 

 upon the types. Being in continual and unchanged existence, 

 they again had their influence, among other conditions, in form- 

 ing the secondary divisions of the type, that is, the groups called 

 orders, and have caused many or all of the deflections of these 

 from the class-lines of structure. In the mammals the order of 

 whales and bats, and in a less degree the ungulates, are cases in 

 point. Among the birds, the ostriches, with the ordinary wad- 

 ing, running, and swimming birds, are examples. Even in the 

 orders the influence of these great primary conditions is not lost, 

 but with less and less power, as the specializations of family, 

 genus, and species are reached, they still show their force in 



