366 PHYSIOLOGICAL DEVELOPMENT. 



continue to perform its function when called upon, just 

 as thougli it were in its place : a fragment of the creature's 

 own body placed in the gullet, will be propelled through it, 

 or swallowed by it. But, as the seeming strangeness of this 

 fact implies, we find no such independent actions of analogous 

 parts in the higher animals. A piece cut out of tho 



disc of a Medusa, continues with great persistence repeating 

 tViose rhythmical contractions which we see in the disc as 

 a whole ; and thus proves to us that the contractile function 

 iiL each portion of the disc, is in great measure independent- 

 Bat 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 a motor apparatus 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 diiferentiated 

 animal than in a more differentiated animal, may be illus- 

 trated 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 

 without injury. Doubtless this difference is proximate)}' due 

 to the greater functional activity in the one case tlian in tho 

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

 duced carbonic acid ; 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 

 tti)paratus has become highly specialized. Here, 



