GENERAL LAW OP ANIMAL FERTILITY. 589 



ration is effected." * Not iinfreqnently in the Nais tliis process is 

 repeated in the young one before it becomes independent of the 

 parent. The higher Annelida are distinguished by the greater 

 number of segments held in continuity ; an obvious result of 

 comparatively infrequent fission. In the class Myriapoda, which 

 stands next above, " there is no known instance of multiplication 

 by fission." f Yet even here the law may be traced both in the 

 number and structure of the segments. The length of the body 

 is still increased after birth " by gemmation from (or partial 

 fission of) the penultimate segment." The lower members of the 

 class are distinguished from the higher by the greater extent to 

 which this gemmation is carried. Moreover, the growing aggre- 

 gative tendency is seen in the fact, that each segment of the Julus 

 " is formed by the coalescence of two original segments," J whilst 

 in the Scolopendridce, which are the highest of this class, " the 

 head, according to Mr. Newport, is composed of eight segments, 

 which are often consolidated into one piece ; " § both of which 

 phenomena may be understood as arrests of that process of fission, 

 which, if allowed to go a little further, would have produced dis- 

 tinct segments ; and, if allowed to go further still, would have 

 separated these segments into groups. 



§ 8. Remarking, first, how gradually this mode of multiplica- 

 tion disappears — how there are some creatures that spontaneously 

 divide or not according to circumstances ; others that divide 

 when in danger (the several parts being capable of growing into 

 complete individuals) ; others which, though not self -dividing, can 

 live on in each half if artificially divided ; and others in which 

 only one of the divided halves can live — how, again, in the Crus- 

 taceans the power is limited to the reproduction of lost limbs ; 

 how there are certain reptiles that can re-supply a lost tail, but 

 only imperfectly ; and how amongst the higher Vertebrata the 

 ability to repair small injuries is all that remains — remarking thus 

 much, let us now, by way of preparation for what is to follow, 

 consider the significance of the foregoing facts taken in connec- 

 tion with the definition of Life awhile since given. 



This spontaneous fission, which we have seen to be, in all 

 cases, more or less destructive of individual life, is simply a 

 cessation in the co-ordination of actions. From the single cell, 

 the halves of whose nucleus, instead of continuing to act together, 

 begin to repel each other, fly apart, establish distinct centres of 

 assimilation, and finally cause the cell to divide ; up to the Anne- 

 lidan, whose string of segments separates, after reaching a certain 

 length ; we everywhere see the phenomenon to be fundamentally 



* Prin. ofPh>/s., p. 954. + Ibid., p. 968. 



: /i/(/, p. 088. § /6m/., p. 958. 



