ECOLOGICAL 117 



retract forcibly. Thus in his Naturalist's Voyage, Darwin wrote of 

 Stylatula: "At low water, hundreds of these zoophytes might be 

 seen projecting like stubble, with the truncate end upwards, a few 

 inches above the surface of the muddy sand. When touched or 

 pulled they suddenly drew themselves in with force so as nearly 

 or quite to disappear." (6) A higher level has been reached by many 

 of the simpler free-swimming Siphonophores, such as the beautiful 

 bluish Velella, fleets of which are sometimes seen in the Mediter- 

 ranean, each with a vertical triangular sail rising above the surface. 

 Here and elsewhere there is division of labour and unified locomotion. 

 The "Fire-Flame" Tunicate illustrates the same level of integrated 

 locomotion. (7) Highest of all and very striking are the most com- 

 plicated Siphonophores with hundreds of polymorphic individuals, 

 yet so well integrated by their nervous system that the colony 

 swims as if it were a slow-going fish. It is a remarkable Natural 

 History fact that a Portuguese Man-of-War, a group-unity of many 

 small members, can capture a mackerel that comes against it. But 

 our general point is simply that animal colonies illustrate a gradual 

 inclined plane from aggregates to integrates. 



5. One line of integrative evolution ends in free-swimming colo- 

 nies such as the Portuguese Man-of-War and the Pyrosome. A second 

 line is that of the instinctive societies that find a climax in the ant- 

 hill, the beehive, and the termitary. They obviously differ from the 

 colonies we have discussed in being physically discontinuous. The 

 bonds are psychical rather than physiological, but they vary greatly 

 in their subtlety. It is a noteworthy fact, vouched for by that skilled 

 observer, Wheeler, that social habits have arisen among insects no 

 fewer than twenty-four different times ; and this number will prob- 

 ably be added to as our knowledge of tropical insects grows. This fact 

 is enough of itself to indicate the strength of the organic trend 

 towards co-operation or sociality. But there are many diverse 

 societary forms: "some of them are small and depauperate, mere 

 rudiments of societies, some are extremely populous and present 

 great differentiation and specialisation of their members, whereas 

 others show intermediate conditions." Our first question must be 

 in regard to their common features. 



{a) A society of termites may include many thousands of indi- 

 viduals, but they are all descended from a pair of "founders". This is 

 the typical state of affairs among social insects, though there are 

 some large ant-hills that include several queens, each with her own 

 abundant progeny. In the transient summer colony of the Humble- 

 bees some of the workers may lay eggs, which, being unfertilised, 

 develop into drones, so that for a short time there are three genera- 

 tions together; and this may occur elsewhere. But the typical com- 

 munity among ants, bees, and wasps consists of a queen and a large 

 body of offspring; the great majority being arrested females or 



