96 MORPHOLOGICAL DEVELOPMENT. 



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 successive 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 

 therefore is not calculated to produce among them any 

 structural multiformity. For the same reason no marked 

 physiological division of labour arises among them; and 

 consequently no combination close enough to disguise their 

 several individualities. 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 them to utilize one another's functions. And 

 hence, while the originally-like individuals are rendered 

 unlike, they will have their homologies further obscured by 

 their progressing fusion into an aggregate individual of a 

 higher order. 



These converse conditions are in nearly all cases fulfilled 

 where the successive individuals arising by continuous devel- 

 opment are so budded-off as to form a linear series. I say 

 in nearly all cases, because there are some types in which 

 the associated individuals, though joined in single file, are 



