100 ANIMAL INDIVIDUALITY [CH. 



if living solitary. Colonial life in such species (which 

 are always sedentary), appears to be merely a device 

 for making the fullest use of a place with good food- 

 supply. Such spots are few and far between, and are 

 discovered by rare individuals only ; thus it is of 

 advantage to retain the descendants of these favoured 

 few bound together there in colonies rather than 

 send them off at once into the world with more 

 chances of failure than of success. 



In other colonies, function is not so diffuse, and 

 there is a function of the whole which is more than, 

 and sometimes quite different from, the sum of the 

 separate functions of the parts. Even in sedentary 

 species this can sometimes be seen ; in Zoothamnium, 

 a colonial bell-animalcule, for instance, a touch on a 

 single one of the animals composing the colony causes 

 the whole colony to retract out of harm's way. This 

 general contraction, common to a number of in- 

 dividuals, though by no means a necessary result of 

 colonial life, could obviously not occur if the in- 

 dividuals were living separately, however closely 

 they were crowded side by side. But it is in free- 

 swimming colonies that the unity of common function 

 is most pronounced. To take the simplest possible 

 example, imagine two actively-swimming protozoa of 

 the same species joined together by whatever means 

 you please. If free, each would have a similar motion 

 to the other, but both would be independent. When 



