88 MORPHOLOGICAL DEVELOPMENT. 



above described, in which the united animals maintain their 

 individualities so distinctly that the individuality of the 

 aggregate remains vague, are constructed in such ways that 

 the united animals carry on their several activities with 

 scarcely any mutual hindrance. The members of a branched 

 Hydrozoon such as is shown in Fig. 149 or Fig. 150, are so 

 placed that they can all spread their tentacles and catch 

 their prey as well as though separately attached to stones or 

 weeds. Packed side by side on a flat surface or forming a tree- 

 like assemblage, the associated individuals among the Polyzoa 

 are not unequally conditioned ; or if one has some advantage 

 over another in a particular case, the mode of growth and 

 the relations to surrounding objects are so irregular as to 

 prevent this advantage re-appearing with constancy in suc- 

 cessive generations. Similarly with the Ascidians growing 

 from a stolon or those forming an annular cluster : each of 

 them is as well placed as every other for drawing in the 

 currents of sea-water from which it selects its food. In 

 these cases the mode of aggregation does not expose the 

 united individuals to multiform circumstances ; and there- 

 fore is not calculated to produce among them any structural 

 multiformity. For the same reason no marked physiologi- 

 cal division of labour arises among them ; and consequently 

 no combination close enough to disguise their several indi- 

 vidualities. But under converse conditions we may expect 

 converse results. If there is a mode of integration which 

 necessarily subjects the united individuals to unlike sets of 

 incident forces, and does this with complete uniformity from 

 generation to generation, it is to be inferred that the united 

 individuals will become unlike. They will severally assume 

 such different functions as their different positions enable 

 them respectively to carry on with the greatest advantage to 

 the assemblage. This heterogeneity of function arising 

 among them, will be followed by heterogeneity of structure ; 

 as also by that closer combination which the better enables 

 ihem to utilize one another's functions. And hence, whilo 



