588 APPENDIX A. 



forms is similarly cliaracterized by increase of the aggrec^ative 

 tendency or diminution of the separative, and similarly exhibits 

 the necessary antagonism between the development of the in- 

 dividual and the increase of the race. That process of grouping 

 which constitutes the first step towards the production of complex 

 organisms, we shall now find repeated in the formation of series 

 of groups. Just as a diminution of the separative tendency is 

 shown in the aggregation of divided monads, so is & further 

 diminution of it shov/n in the aggregation of the divided groups 

 of monads. The first instance that occurs is afforded by the 

 compound polypes. " Some of the simpler forms of the com- 

 posite Hydroida^'''' says Carpenter, " may be likened to a Hydra, 

 whose gemmae, instead of becoming detached, remain perma- 

 nently connected with the parent ; and as these in their turn may 

 develop gemmae from their own bodies, a structure of more or 

 less arborescent character may be produced." * A similar species 

 of combination is observable amongst the Bryozoa, and the com- 

 pound Tunicata. Every degree of union may be found amongst 

 these associated organisms ; from the one extreme in which 

 the individuals can exist as well apart as together, to the other 

 extreme in which the individuals are lost in the general mass. 

 Whilst each Bryozoon is tolerably independent of its neighbour, 

 "in the compound Hydroida, the lives of the polypes are sub- 

 ordinate to that of the polypdom." f Of the Salpidoe and 

 Pyrosomidce, Carpenter says : — " Although closely attached to one 

 another, these associated animals are capable of being separated 

 by a smart shock applied to the sides of the vessel in which they 

 are swimming. ... In other species, however, the sepa- 

 rate animals are imbedded in a gelatinous mass," and in one 

 kind " there is an absolute union between the vascular systems 

 of the different individuals." X 



In the same manner that with a given aggregative tendency 

 there is a limit to the size of groups, so is there a similarly- 

 determined limit to the size of series of groups ; and that 

 spontaneous fission which we have seen in cells and groups of 

 cells we here find repeated. In the lower Annelida, for example, 

 " after the number of segments in the body has been greatly 

 multiplied by gemmation, a separation of those of the posterior 

 portion begins to take place ; a constriction forms itself about the 

 beo-inning of the posterior third of the body, in front of w^hich 

 the alimentary canal undergoes a dilatation, whilst on the seg- 

 ment behind it a proboscis and eyes are developed, so as to form 

 the head of the young animal which is to be budded off ; and in 

 due time, by the narrowing of the constriction, a complete sepa- 



* Prin. of Pht/s., p. 267. + Ibid., p. 2*76. 



X Ibid., 2nd edit., p. 115. 



