12(5 THE VOYAGE OF H.M.S, CHALLENGEE. 



destruction is only partial, the other homonodynamic portiona temporarily ministering, 

 thanks to their more independent relation to the injured region. 



When the faculty of repair of damage, occasioned by the severing of the animal into 

 two or more portions, has in the course of generations become more and more complete, it 

 can be readily understood that it becomes at the same time a defensive instead of being 

 only a curative process. An animal that at the approach of danger can separate in two or 

 more parts, each of them capable of reproducing an entire new animal, evades this danger 

 very effectively by doing so ; whereas another that is attacked in the same way and does 

 not possess this faculty, is laid hold of, shaken about, and wholly or partly swallowed. 

 So in the Nemertea there is indeed a very strong faculty of spontaneous division 

 combined with the faculty of repair ^ ; and anybody who has observed a fresh and living 

 Cerehratulus, wdth its extremely delicate sense of touch, commence to rupture into two, 

 in preference at the spot where it was grasped with the forceps, cannot fail to see in this 

 a defensive action. 



This mode of self-defence may in quite another respect be useful to the species, 

 because at the same time it serves for propagation. Thus we see that the passage of 

 this defensive process to that of reproduction by fission is so gradual, that it would be 

 impossible to decide in every case what name should properly be applied to it. It cannot 

 well be denied that in all probability ours is only a special case, in which the power of 

 reproducing the species by a process of fission, reaching down as far as the unicellular 

 ancestors, has come to be regulated by other motor forces than growth and — if it may 

 not be called voluntary fission — still may be regarded as sudden and spontaneous fission, 

 brought about by external influences, of a threatening nature to the further existence of the 

 specimen. This regulation is no doubt a consequence of selection. Schizogony having 

 once been established, it must have been further beneficial to the species, on the grounds 

 that were developed above, that the internal organs should be present in multiple num- 

 bers, and this having once come about it is easy to understand, that a regular, rigorously 

 metamerous arrangement of this multiple material, still more fuUy answers the same 

 purpose, and is gradually evolved under the influence of selection. 



Thus we may be said to be able to follow the appearance of metamery in a bilateral 

 animal, along aU the gradual steps by which it is evolved, and many of these steps have 

 remained fixed and permanent in different Nemertean genera. 



The last system that will participate in this metamery is the muscular system, and a 

 rash conclusion — such as is not rare in these days of ontogenetic fetichism — might lead 

 to the rejection of the views concerning metamery here developed, on the consideration 

 that it is exactly the metamery of the muscular system which appears first of all in the 



^ Both 51'Intosli and Barrois have observed and described very peculiar cases of repair in Nemertea, where the 

 head, brain, side-organs, &c., were reproduced on a headless trunk-piece. These experiments are well worthy of careful 

 repetition. It mijjht be that only those fragments in which a portion of the oesophagus was retained were capable of 

 repair of the head. 



