42 Relation of Animal Motion to Animal Evolution. [January, 
r 
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. 
In fine, it requires little argument to show that the environ- 
ment has had in the past as in the present, a primary influence 
over the movements of animals. 
II. 
I will now endeavor to exhibit some reasons for believing that 
the movements of animals affect their structure directly. 
There are two alternative propositions expressive of the rela- 
tions of the structures of animals to their uses. Either the use 
or attempt to use ‘preceded the adaptive structure, or else the 
structure preceded and gave origin to the use. The third alter- 
native, that use and structure came into being independently of 
each other is too improbable for consideration in the present 
article. Many facts render the first of these propositions much 
the more probable of the two. 
A general ground for suspecting that movement affects struc- 
ture is the fact well known to systematic zodlogists, that adaptive _ 
characters are the least reliable in systematic classification, £. €. 
are the most variable. What we call adaptive characters are 
those whose teleological significance we can most easily perceive; 
those whose uses are at the present time most obvious. System- 
atists habitually fall back on characters which are apparently the 
least related to the ordinary necessities of the life of the animal, — 
and this not from any theoretical considerations, but because such 4 
characters are found to be the most constant; this is a very sig- i 
nificant fact, showing as it does that it is the adaptive structures 
-= which are undergoing modification to-day. And this truth can 
doubtless be discerned in all past ages, for many of the structures 
which are not now more related to the needs of an animal than 
many others might be, were at one time most essential to its well- l 
being, or necessarily related to its environment. Such are the — 
structural characters of the heart and arteries already enumerated. 
There seems to be no reason why all Vertebrata might not exist 
with equal comfort and success at the present if possessed of 4 
