12 EVOLUTION BY ATROPHY 



equivalent, and each retains all the functions of 

 life (Spirogyra, Choanoflagellates, Hydra). The 

 colony may disintegrate, but the cells or indi- 

 viduals set at liberty are capable of living alone, 

 and do not form a society. 1 



As the organisation of a colony becomes more 

 complicated, the units proceed to differentiate. Each 

 one assumes certain functions in particular, and 

 becomes less adapted to discharge others. If such 

 a colony is dispersed, the individuals are incapable 

 of maintaining a separate existence. 2 



1 (a) A thread of Spiroc/yra consists of cells placed end to end, 

 physiologically connected with one another. Under certain 

 conditions, however, quite apart from the phenomenon of cellular 

 reproduction, all these cells are capable of isolating themselves. 

 The colony is thus transformed into a set of individuals capable 

 of maintaining a separate existence, and no longer in connection 

 with one another. 



(b) Among the Choanoflagellates, some consist of free individuals, 

 while in others all the individuals are united by a common stalk, 

 and intercommunicate by protoplasmic threads in the stalk. When, 

 for some cause, these individuals separate, they never form them- 

 selves into a society. 



(c) A colony of Hydra is formed by the budding of a single indi- 

 vidual. While nourishment is abundant, all the individuals of the 

 colony retain their connection with each other, and may themselves 

 give rise to buds. On the other hand, when food is scarce, the 

 colony disintegrates and each individual lives a free life without 

 entering into social relations with its neighbours. 



2 In some cases the terms individual and colony become ex- 

 tremely involved. Although one accept our view, it is difficult to 

 rigorously apply our generalizations to such, facts as the following : 

 Certain male cephalopods, at the period of reproduction, separate 

 from their bodies a specialized tentacle (hectocotyle) in which is 

 stored the seminal fluid. This organ, set at liberty, swims in the 



