RELATION OF ANIMAL MOTION TO ANIMAL EVOLUTION. 351 



of plants and animals is clear enough. The potency of such 

 changes may be read in the physical history of the earth. A long 

 series of modifications preceded the advent of life upon it, and 

 change, both gradual and sudden, has been exhibited in the con- 

 figuration and climate of all portions of the surface of the globe 

 since that period. Animals have again and again been called 

 upon to face new conditions, and myriads of species have fallen 

 victims to the inflexibility of their organization which has pre- 

 vented adaptation to new surroundings. But it is evident that if 

 change of environment has had any influence in the progress of 

 evolution, it has not been alone destructive. It has preceded life 

 as well as death, and has furnished the stimulus to beings capable 

 of change, while it has destroyed those which were incapable of 

 it. It is a truism that change of physical conditions has preceded 

 all great faunal changes, and that the necessity for new mechanism 

 on the part of animals has always preceded the appearance of new 

 structure in geologic times. 



The embryology and paleontology of vertebrated animals show 

 that the primary steps in the progress of this branch of the animal 

 kingdom are marked by the successive changes in the structure of 

 the circulatory system. First we have the various mechanical 

 methods for the aeration of blood in a watery medium ; the result 

 being a fluid whose metamorphosis in nutrition produces no heat. 

 After the fishes followed Batrachia, the earliest air-breathers, 

 whose long tarriance to-day in early aquatic stages is an epitome 

 of the necessarily *' amphibious" character of air-breathing verte- 

 brate life, when land and fresh water, in constantly changing 

 areas, were rising and separating from the universal ocean. The 

 successive disappearances of the traces of the fish type of circula- 

 tion in Batrachia and reptiles, are familiar facts ; and the exclu- 

 sion of the unaerated blood from the systemic circulation in the 

 birds and mammals marks the increase of general temperature 

 which gives those classes one of their claims to sui)eriority. 



The appearance of land of course furnished the opportunity 

 for aquatic animals to assume a terrestrial life. Marine animals 

 which had acquired the habit of gulping air from the surface, 

 which some of them now possess, perhaps because its richness in 

 oxygen produced an agreeable exaltation or intoxication, would 

 not find visits to the land difficult. And this would naturally 

 follow the necessity of escape from aquatic enemies, or the search 

 for new supplies of food. 



