106 MORPHOLOGICAL DEVELOPMENT. 



animals in general are produced. As among plants, and as 

 among demonstrably-compound animals, we see that the only 

 thing required for the formation of a permanent chain of 

 gemmiparously-produced individuals, is that by remaining 

 associated such individuals will have advantages greater than 

 are to be gained by separation. Further, comparisons of 

 the annuloid and lower annulose forms, disclose a number 

 of those transitional phases of integration which the hypo- 

 thesis leads us to expect. And, lastly, the differences among 

 these united individuals or successive segments, are not 

 greater than the differences in their positions and functions 

 explain — not greater than such differences are known to pro- 

 duce among other united individuals: witness sundry com- 

 pound Hydrozoa. 



Indirect evidence of much weight has still to be givei 

 Thus far we have considered only the less-developed Annu- 

 losa. The more integrated and more differentiated types of 

 the class remain. If in them we find a carrying further of 

 the processes by which the lower types are here supposed to 

 have been evolved, we shall have additional reason for be- 

 lieving them to have been so evolved. If we find that in 

 these superior orders, the individualities of the united seg- 

 ments are much less pronounced than in the inferior, we 

 shall have grounds for suspecting that in the inferior the 

 individualities of the segments are less pronounced than in 

 those lost forms which initiated the annulose sub-kingdom. 



[Note. — Partly from the wish to incorporate further evi- 

 dence, and partly from the wish to present the evidence, old 

 and new, in a more effective order, I decide here to recast the 

 foregoing exposition. 



Significant traits of development are exhibited in common 

 by two groups otherwise unallied — certain of the Platyhel- 

 minthes and certain of the lower Annulosa. Of the Platyhel- 

 minthes the ordinary type is an unsegmented creature: a 



