148 ANIMAL INDIVIDUALITY [CH. 



a complete sponge ; here aggregate differentiation has 

 been at work, and whole cells and tissues are affected 

 instead of parts of cells. 



The second narrowing factor is harder to precise ; 

 but though we do not know its exact nature, we can 

 often see it at work. There are many animals, such 

 as man himself, where regeneration is almost non- 

 existent although in any given case all the necessary 

 substances and kinds of tissue would appear to be 

 present. Here the failure to regenerate seems to 

 stand in some general relation with the degree of 

 specialization of the tissues; most animals can 

 regenerate more completely when young or embryonic 

 than when they are grown up. 



The third factor is more obvious : certain bits of 

 organic machinery are of such a nature that it is 

 physically impossible for the animal to live at all if 

 they are seriously tampered with. It is just because 

 our blood-circulation is so swift and efficient and our 

 nervous system so splendidly centralized that damage 

 to heart or brain means almost instant death to us, 

 while a brainless frog will live for long, and a heart- 

 less part of a worm not only live but regenerate. 

 Thus here again sacrifice is at the root of progress, 

 and only by surrendering its powers of regeneration 

 and reconstitution has life been able to achieve high 

 individualities with the materials allotted her. 



But this original property of living matter is 



