92 MORPHOLOGICAL DEVELOPMENT. 



self a certain unit of composition for all creatures of a par- 

 ticular class, and then displayed his ingenuity in building up 

 a great variety of forms without departing from the " arche- 

 typal idea." On the other hand, examination made it mani- 

 fest that even were such a conception worthy of being enter- 

 tained, it would have to be relinquished ; since in each class 

 there are numerous deviations from the supposed " archetypal 

 idea." Still less can these traits of structure be accounted 

 for teleologically. That certain organs of nutrition and re- 

 spiration and locomotion are repeated in each segment of a 

 dorsibranchiate annelid, may be regarded as functionally ad- 

 vantageous for a creature following- its mode of life. But 

 why should there be a hundred or even two hundred pairs of 

 ovaries? This is an arrangement at variance with that 

 physiological division of labour which every organism pro- 

 fits by is a less advantageous arrangement than might have 

 been adopted. That is to say, the hypothesis of a designed 

 adaptation fails to explain the facts. Contrariwise, 



these structural traits are just such as might naturally be 

 looked for, if these annulose forms have arisen by the in- 

 tegration of simpler forms. Among the various compound 

 animals already glanced at, it is very general for the united 

 individuals to repeat one another in all their parts repro- 

 ductive organs included. Hence if, instead of a clustered or 

 branched integration, such as the Caclentcrata and Mollmcoida 

 exhibit, there occurs a longitudinal integration ; we may ex- 

 pect that the united individuals will habitually indicate their 

 original independence by severally bearing germ-producing 

 or sperm-producing organs. 



The reasons for believing one of these creatures to be an 

 aggregate of the third order, are greatly strengthened when 

 we turn from the adult structure to the mode of develop- 

 ment. Among the Dorsibranchiata and Tiibicola, the em- 

 bryo leaves the egg in the shape of a ciliated gemmule, not 

 much more differentiated than that of a polype. As shown 

 in Fig. 162, it is a nearly globular mass ; and its interior 



