374 PHYSIOLOGICAL DEVELOPMENT. 



to perform its function when called upon, just as though it 

 were in its place: a fragment of the creature's own body 

 placed in the gullet, will be propelled through it, or swal- 

 lowed by it. But, as the seeming strangeness of this fact 

 implies, we find no such independent actions of analogous 

 parts in the higher animals. Again, a piece cut out of the 

 disc of a Medusa continues with great persistence repeating 

 those rhythmical contractions which we see in the disc as a 

 whole; and thus proves to us that the contractile function 

 in each portion of the disc, is in great measure independent. 

 But it is not so with the locomotive organs of more differen- 

 tiated types. When separated from the rest these lose their 

 powers of movement. The only member of a vertebrate 

 animal which continues to act after detachment, is the heart ; 

 and the heart has motor powers complete within itself. 



Where there is this small dependence of each part upon 

 the whole, there is but small dependence of the whole upon 

 each part. The longer time which it takes for the arrest of 

 a function to produce death in a less-differentiated animal 

 than in a more-differentiated animal, may be illustrated by 

 the case of respiration. Suffocation in a man speedily 

 causes resistance to the passage of the blood through the 

 capillaries, followed by congestion and stoppage of the heart : 

 great disturbance throughout the system results in a few 

 seconds, and in a minute or two all the functions cease. 

 But in a frog, with its undeveloped respiratory organ, and a 

 skin through which a considerable aeration of the blood is 

 carried on, breathing may be suspended for a long time with- 

 out injury. Doubtless this difference is proximately due to 

 the greater functional activity in the one case than in the 

 other, and the more pressing need for discharging the pro- 

 duced carbon dioxide; but the greater functional activity being 

 itself made possible by the higher specialization of functions, 

 this remains the primary cause of the greater dependence of 

 the other functions on respiration, where the respiratory 

 apparatus has become highly specialized. Here 



