352 MECHANICAL EVOLUTION. 



In fine, it requires little argument to sliow that the enyiron- 

 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 struct- 

 ure is the fact well known to systematic zoologists, that adaptive 

 characters are the least reliable in systematic classification, i. e., 

 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 

 characters are found to be the most constant. This is a very sig- 

 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- 

 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 Vertehrata might not exist 

 with equal comfort and success at the present if possessed of a 

 uniform organization in this respect. But the successive modi- 

 fications which they present were, in past ages, most intimately 

 connected with the progressive changes of the medium in which 

 they lived, as to the volume of oxygen su23plied for respiration, 

 as compared with that of the vapor of water, carbonic acid gas, 

 etc. But it must be here noted, in passing, that there are many 

 structures in animals which have never been adaptive, but which 



