1878.] Relation of Animal Motion to Animal Evolution. 41 
to the fact that such modifications must be realized during a lim- 
ited portion of the life of an animal at least; that is, during the 
period of growth, when it is not at all or but little subject to the 
influence of external environment, but is usually protected or 
supported by the parent. 
That the environment and changes in it affect the movements 
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 capa- 
ble of change, while it has destroyed those which were incapable 
of it. It is a truism that change of physical conditions has pre- 
ceded 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 aération of blood in a watery medium; 
the result being a fluid whose metamorphosis in nutrition pro- 
as duces 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 vertebrate life, when land and fresh water, in constantly 
changing areas, were rising and separating from the universal 
ocean. The successive disappearance of the traces of the fish 
type of circulation in Batrachia and reptiles, are familiar facts; and 
the exclusion of the unaérated blood from the systemic circula- 
tion in the birds and mammals marks the increase of general 
temperature which gives those classes one of their ciamis to 
superiority. : 
_ The appearance of land of course furnished the opportunity 
