464 LAWS OF MULTIPLICATION. 



kindred but more-organized types, the gemmiparity is 

 greatly restricted, or disappears. Among the free-swimming 

 Hydrozoa, 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 Hydra 

 and the compound Hydroids. These last, along with more- 

 differentiated tissues, show us a gemmation which does not 

 go on all over the body of each polype, and much of it does 

 not end in separation. 



It is, however, among the Annulosa that progressing 

 organization is most conspicuously operative in diminishing 

 agamogenesis. The segments or " somites " composing 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 types 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 ad- 

 vance 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. 



Phaenogams that have but little supporting tissue may 

 fairly be classed as structurally inferior to those having stems 

 with a bulky and complex woody system; for these imply 



