DEVELOPMENT AND GENESIS. 44^ 



power to produce young ones from neai'ly all parts of its 

 body, is due to the comparative homogeneity of its body. In 

 kindred but more-organized types, the gemmiparity is 

 greatly restricted, or disappears. Among the free-swimming 

 Hydrazoa, multiplication by budding, when it occurs at all, 

 occurs only at special places. That increase of structure 

 apart from increase of size, is here a cause of declining agamo- 

 genesis, we may see in the contrast between the simple and 

 the compound Hydroida ; which last, along with more- 

 differentiated tissues, show us a gemmation which does not 

 go on all over the body of each polype, and mujh of which 

 does not end in separation. 



It is, however, among the A.nnulosa that progressing 

 organization is most conspicuously operative in diminishing 

 agaraogenesis. The segments or " somites " that compose an 

 animal belonging to this class, are primordially alike ; and, 

 as before argued (§§ 205-7), are probably the homologues of 

 what were originally independent individuals. The progress 

 from the lower to the higher tj'pes of the class, is at once a 

 progress towards types in which the strings of segments cease 

 to undergo subdivision, and towards types in which the seg- 

 ments, no longer alike in their structures and functions, have 

 become physiologically integrated or mutually dependent. 

 Already this group of cases has been named as illustrating 

 the antagonism between growth and asexual genesis ; but it 

 is proper also to name it here; since, on the one hand, the 

 greater size due to the ceasing of fission, is made possible only 

 by the specialization of parts and the development of a co- 

 ordinating apparatus to combine their actions, and since, on 

 the other hand, specialization and co-ordination can advance 

 only in proportion as fission ceases. 



§ 346. The inverse variation of development and sexual 

 genesis is by no means easy to follow. One or two facts indi- 

 cative of it may, however, be named. 



PhaRnogaras that have but little supporting tissue may 



