VIII NATS PROBOSCIDEA 391 



Cases belonging to the second of the above-defined groups 

 are much the more important for my argument, for in tliem 

 the recrescence is evidently entirely the result of the definitely 

 directed powers acquired by the ancestors and transmitted by 

 them to their descendants. 



The processes which underlie recrescence are no others than 

 those which condition asexual multiplication. Indeed, in 

 lower animals, where recrescence leads to tlie production 

 of entire animals, the two phenomena coincide in every 

 respect. Whether we cut a small annelid, a water-worm 

 (Nais proboscidea), into two parts, with the result that each 

 part grows into a new perfect individual, or whether the worm 

 divides itself spontaneously and grows into two new animals, 

 in both cases we have exactly the same process. 



In the recrescence of the water- worm we have an example 

 of the second kind of recrescence ; there is no external 

 stimulus which directly prompts it. Of the relation of 

 nutrition to the process I say nothing — the possible objec- 

 tions to be made on this ground are obvious of themselves, as 

 also the considerations by which they can be extenuated or 

 overcome. The parts of the worm, without any special 

 external stimulation during the process, grow in a definite 

 direction in a perfectly constant manner again into an entire 

 animal— that is for me the principal fact. New eyes and a 

 new tentacle are produced on the anterior segment, and a new 

 nerve-ring and cerebral ganglia within it. From this anterior 

 segment, by its repeated division, the longitudinal growth of 

 the new animal takes place, that is, from before backwards, 

 just as in natural division. This recrescence evidently repeats 

 the process which originally led to the formation of the worm 

 with its eyes and proboscis, when it was evolved from a uniseg- 



of internal and external forces on the formation of their organs, see Pfliiger's 

 Archiv f. Physiol. Bd. xv. (Extract from Ueher Organhildung im Pjlanzenrexch, 

 I. Bonn, 1878.) 



