VIII CAUSE OF RECRESCENCE 393 



the legs and claws of Crustacea, the tentacles of snails, and 

 even the heads of the latter, if the brain is not destroyed, 

 are reproduced is well known, and has already been partly 

 discussed. Equally well known is the recrescence of the 

 arms of star-fishes. The leajs of insects also sfrow acrain. I 

 have only brought together here a few out of the large number 

 of instances of recrescence. 



How then does it come about that such organs, many of 

 them exceedingly complex, grow again from the body at tlie 

 places where they were originally present, without the direct 

 action of stimuli ? 



The old explanation was that it was due to the "formative 

 tendency," and this was deemed sufficient. We may still use 

 the same term, provided that we get rid of the meaning 

 which the earlier school attached to it, namely that of a 

 spontaneous impulse more or less independent of matter. If 

 we guard against this, if there be any need to do so, and 

 employ the term in the sense of the action of definitely 

 directed forces contained in the material of the organism, 

 then there is nothing to be said against it. We apply the 

 word " shoots " in an entirely passive sense to parts of plants. 

 And in the same sense recrescence consists in the production 

 of shoots, of normally- constructed parts which are, as it were, 

 shot out from the organism by natural necessity. Thus we 

 may use the word tendency or impulse in an active sense, 

 though with certain limitations, in speaking of recrescence. 

 But we conceive the formative tendency not as an independ- 

 ent force by the action of which everything is explained, but 

 as a force of which we have to discover the causes. 



My view is that the recrescence of lost parts of tlie 

 organism must be ascribed to heredity, that the latter is tlie 



hesonders AmphiUen und Reptilien, Cassel and Berlin, 1885. In reptiles 

 (lizards and geckos) the recrescence of the spinal cord takes place only 



imperfectly. 



