SELF-DIVISION AND REGENERATION 



culature of the body, would effectively prevent the self-division of 

 the animal. 



The fifth question has many sides. It involves us on the one 

 hand in a historical question of the origin of self-division, and on the 

 other hand in a discussion of the stimulus that brings about, not only 

 the division, but the changes that precede the division in those cases 

 in which the new part develops before division takes place. 



Several zoologists have held that the process of self-division fol- 

 lowed by regeneration has been the starting-point for the process of 

 propagation preceded by regeneration. Von Kennel, for instance, 

 maintains that self-division in some of the annelids has arisen in this 

 way. He says : " We recognize everywhere in the animal kingdom the 

 power of organisms to replace lost parts, and we call this regenera- 

 tion. It may be developed in very different degrees in animals, and, 

 as a rule, only those parts of the body have the power of regenera- 

 tion that still possess the organs that are essential for independent 

 existence. The higher the organization of the animal, so much the 

 less is its power of regeneration, perhaps, because the division of 

 labor of the different organs has gone so far that extensive injuries 

 cannot be repaired. . . . There is no doubt that this power is adap- 

 tive, in a high degree, to preserve the species under unfavorable con- 

 ditions, so that they are much better off in the battle for existence 

 than are the animals that live under the same conditions but have 

 not the power of regeneration. . . . The power of regeneration that 

 gives the animal a better chance in the battle for existence and, there- 

 fore, makes more certain the continuance and the distribution of the 

 species will be, as is well known from numerous observations, in a 

 high degree inherited, indeed even increased so that its descendants 

 will possess that power in a higher degree than their forefathers ; and, 

 in consequence, a much smaller stimulus (motive) suffices, than at 

 first, to bring about the division of the parts." After showing, accord- 

 ing to the usual formula, that the process of regeneration is useful, 

 and, therefore, would come under the guidance of natural selection, 

 von Kennel proceeds to show how the result is connected with an 

 external stimulus! He asks: "Can accidental injuries account for 

 the result (viz. for the division in lumbriculus, planarians, and star- 

 fish), since how few starfish are there with regenerating arms in com- 

 parison with the enormous number of uninjured individuals? Should 

 we not rather look for the external stimuli that have initiated the pro- 

 cess of self-division ? " " Animals that have developed the power of 

 regeneration by a long process of inheritance will have acquired along 

 with this the property of easier reaction to all external adverse condi- 

 tions. In a sense the sensitiveness for such stimuli is sharpened, and 

 the animal responds at once by breaking up. In the same way the 



