examples of swimming and running birds without flight, and of marine and aërial mammals, have already been given. In these cases, the fundamental structures become in part worn and mutilated, 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 departed from the environment of its ancestors.
But though deflection and antagonism of structures is possible 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 modifications 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 profound modifying influence. Among these are the different mediums respired, air and water, and the different horizons or locations requiring peculiar organs of locomotion, deep and shallow water, earth, rocks, and trees, and air. The divisions of types which are usually dignified by systematists with the title of classes have their reason for existence in conditions of this kind. The five recognized classes of vertebrates—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 forming 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 wading, 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
