multiplication by fission 151 



4. The Phylogeny of the Process of Multiplication 



BY Fission 



There can be little doubt that the process of spontaneous 

 division which occurs in flat-worms and in annelids is to be 

 derived phylogenetically from regeneration, as Kennel * has 

 recently attempted to prove. He has rightly, I believe, shown 

 that multiplication by a spontaneous separation into parts, such 

 as occurs regularly in the freshwater worm Ljuubriciilus , must 

 be looked upon as a prelimixiary stage of that kind of fission, 

 accompanied by regeneration, which occurs in the Naida, for 

 instance. The difference between the two processes consists 

 essentially in the fact that in N'ais the separation into parts is 

 preceded and prepared for by the formation of new head- and 

 tail-ends, which appear between the old segments at the point 

 at which the separation is to take place. Such a preparatory 

 process does not occur in Lu7nbriculns : the region in which 

 division will take place in this worm cannot previously be dis- 

 tinguished, and the new head- and tail-ends are formed subse- 

 quently, after the division has occurred. 



The capacity for division of an individual into parts must 

 naturally be looked upon as an adaptation, and it presupposes 

 some kind of histological and physiological arrangement of 

 which we are at present ignorant. It is, however, quite conceiv- 

 able that when fission had once occurred in a species, it may 

 have been advantageous for a more thorough preparation for the 

 process to take place, and for the structures necessary for the 

 completion of the individuals thus formed to become developed 

 beforehand. Such a capacity for multiplication by spontaneous 

 division necessitates, moreover, the previous possession of the 

 power of regeneration. Hence the latter must have existed in 

 the animal before spontaneous division could take place regularly 

 in the species, and we must thus conclude that the capacity for 

 regenerating portions of the body which had been accidentally 

 torn asunder was first acquired very early in the philogeny of 

 multicellular animals ; and that the special arrangements for 

 multiplication by fission subsequently originated from this 

 capacity for regeneration, and was followed by the formation of 

 new head- and tail-ends. The formation of the new parts pre- 



* J. Kennel, ' Uber Theilung u. Knospung der Thiere,' Dorpat, 1888. 



