24 THE USES AND ORIGIN 



separate stimulus to the polypite, and so of enabling the 

 polypite always to determine which of the whole multi- 

 tude is being stimulated. . . . It is no doubt a 

 benefit to this Medusa that its polypite is able to localize 

 a seat of stimulation in the bell; for the end of the 

 polypite is provided with a stinging apparatus, and is, 

 besides, the mouth of the animal. Consequently, when 

 any living object touches the bell — whether it be an 

 enemy or a creature serving as prey — it must alike be 

 an advantage to the Medusa that its polypite is able to 

 move over quickly to the right spot, in the one case to 

 sting away the enemy, and in the other to capture the 

 prey. - 



It is, in all probability, the delicate impressions pro- 

 duced by contact of the sea-water with the surface of the 

 organism, acting through the intermediation of tlie rudi- 

 mentary ganglia near the edge of the swimming-bell, 

 which tend to incite its apparently * spontaneous ' 

 movements. At all events, when these little bodies are 

 removed the habitual rhythmical contractions of the 

 swimming-bell cease, and a single stimulation of any 

 portion of the bell is then followed by a single contraction. 

 The contrast between the behaviour of such an animal 

 and one which is uninjured, is very striking. f 



Multiply the kind of correlation above typified, and it 

 may be seen that as organisms, or their descendants, 

 increase in tneir ability to discriminate diff'erent impres- 

 sions made upon them from without, so will there grow 

 up muscular responses suitable to each. And the struc- 

 tural modifications, or ' tissues,' through the intervention 

 of which any of these impressions, discriminations, and 

 responses are rendered possible, are no more isolated from 

 others which the creature is capable of receiving or making, 

 * " Nature," vol. xvi. p. 290. f Loc. cit., p. 231). 



