THE MORPHOI-OGICAL COAU'OSITION OF ANIMALS. 89 



the 01 iginally-lLke individuals are rendered unlike, they M'ill 

 havo their homologies further obscured by their progressing 

 fusion into an aggregate individual of a higher order. 



Those converse conditions are in nearly all cases fulfilled 

 where the successive indiAriduals arising by continuous devel- 

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

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

 the associated individuals, though joined in single file, are 

 not thereby rendered very unlike in their relations to the 

 environment ; and therefore do not become differentiated and 

 intefjrated to any considerable extent. I refer to such Asci- 

 diaDS as the Salpidce. These creatures float passively in the 

 sea, attached together in strings. Being placed side by side 

 and having mouths and vents that open laterally, each of 

 them is as well circumstanced as its neighbours for absorb- 

 ing and emitting the surrounding water; nor have the in- 

 dividuals at the two extremities any marked advantages 

 over the rest in these respects. Hence in this type, and in 

 the allied tj-pe Pjjrosoma, which has its component indivi- 

 duals biult into a hollow cylinder, linear aggregation may 

 exist without the minor individualities becoming obscured 

 and the major individuality marked : the conditions under 

 which a differentiation and integration of the component 

 individuals may be expected, are not fulfilled. But where 

 the chain of individuals produced by gemmation, is either 

 habitually fixed to some solid body by one of its extremities 

 or moves actively through the water or over submerged 

 stones and weeds, the several members of the chain become 

 differently conditioned in the way aoove described ; and may 

 therefore be expected to become unlike while they become 

 united. A clear idea of the contrast between these twi 

 linear arrangements and their two diverse results, will be 

 obtained by considering what happens to a row of soldiers, 

 when changed from the ordinary position of a single rank 

 to the position of Indian file. So long as the men stand 

 shoulder to slioulder, they are severally able to use theij 



