6 THE EVOLUTION THEORY 



direction, so that half the tentacles came to lie to the right, the other 

 half to the left, while the body developed between these two groups, 

 so that they became further and further separated from each other, 

 till finally the original transverse axis of the animal became the 

 longitudinal axis. One group of tentacles survived and surrounded 

 the new mouth, while the other at the opposite aboral pole, the new 

 foot, died off. This total change of structure in the polyp, as to the 

 arrangement of its main parts, points to unknown forces, which 

 cannot depend on the determinants as such, but on the vital 

 characters of the living parts, and on the interactions of these with 

 one another. 



The same holds true of all the lower Metazoa that have highly 



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Fig. 96. A Planarian cut transversely into nine pieces. The regeneration 

 of seven of these into entire animals is shown. After Morgan, 



developed regenerative capacity, not only of polyps, but of worms 

 such as the Planarians. Through the experiments of Loeb, Morgan, 

 Voigt, Bickford, and others, we know that these animals respond to 

 almost every mutilation by complete reconstruction, that they may, for 

 instance, as is indicated in Fig. 96, be cut transversely into nine or ten 

 pieces with the result that each of these pieces grows again to a whole 

 animal, unless external influences are unfavourable and prevent it. 



Something similar happens if the head be cut off a Tubularia- 

 polyp, it forms a new head with proboscis and tentacles. It does so, 

 at least, if the stalk of the polyp be left in the normal position ; but 



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