THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 97 



not thereby rendered very unlike in their relations to the 

 environment; and therefore do not become differentiated and 

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

 dians 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 type Pyrosoma, which has its component indivi- 

 duals built 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 above described; and may 

 therefore be expected to become unlike while they become 

 united. A clear idea of the contrast between these two 

 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 shoulder, they are severally able to use their 

 weapons in like ways with like efficiency; and could, if 

 called on, similarly perform various manual processes directly 

 or indirectly conducive to their welfare. But when, on the 

 word of command " right face," they so place themselves 

 that each has one of his neighbours before him and another 

 behind him, nearly all of them become incapacitated for 

 fighting and for many other actions. They can walk or run 

 one after another, so as to produce movement of the file in 

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