REGENERATION. 189 



misleading to speak of the " whole aggregate exerting over 

 its parts a force," etc. 



Rauber himself speaks with much reserve in his comparison 

 between the " regeneration of the crystal" and the regenera- 

 tion of animals and plants. His results show, it seems to me, 

 very clearly that the comparison is only a general analogy; and 

 the moment we attempt to press the comparison further it 

 breaks down at every point. 



It will, no doubt, be admitted by every one at the present 

 time that the form of the crystal is somehow determined by 

 the chemical composition of the substance of which the crystal 

 is made up, and likewise that the form of an animal is also 

 determined by the substances of which it is composed ; but the 

 broken crystal regains its original form only when surrounded 

 by a saturated solution containing the same substance as the 

 crystal. New substance is then added over its entire surface 

 as well as over the part broken off. On the other hand, the 

 process of regeneration is entirely different in an animal or 

 plant. The new material, if any is formed, comes from within 

 the animal or plant. Further, an animal that is slowly starv- 

 ing and decreasing in size will regenerate a missing part. 

 Again, several of the lower forms regenerate by changing over 

 the entire piece into the typical form. Can any one suppose 

 for a moment that such a process is comparable to the re-com- 

 pletion of the form of a crystal ? 



If further evidence is asked, attention may be drawn to 

 those cases in which an organ, different in kind from the one 

 cut off, is regenerated. A striking case of this sort is the 

 regeneration of an antenna in place of an eye, or the develop- 

 ment of a tail at the anterior end of a posterior piece of an 

 earthworm. 



It would not be difficult to multiply at length these points 

 of difference, but what I have said will suffice, I hope, to show 

 how little we can gain by Spencer's comparison. 



Pfliiger l has- given a brief outline of his conception of the 

 process of regeneration. He says, since there is always 



1 Pfliiger, E. Ueber den Einfluss der Schwerkraft auf die Theilung der Zellen 

 und auf die Entwicklung des Embryos, pp. 64-67. 1883. 



