ECOLOGICAL 



"3 



4. Here it may be appropriate to link social animals to those that 

 form colonies physically continuous. Just as in the physical world, 

 where electrons and protons combine to form atoms, and atoms 

 molecules, and molecules micellae in a colloidal state, so in the realm 

 of organisms there are somewhat similar groupings, and regroupings 

 even to colonies. Among the unicellular Protozoa a colony may 

 be formed by continuous budding or by the cohesion of units as 

 they divide. These colonies, e.g. among Radiolarians and Infusorians, 

 are interesting in pointing the way to the origin of a multicellular 

 body. In the beautiful green colony called Volvox, there may be as 

 many as 10,000 cells, forming a hollow ball, and united to one 

 another by delicate protoplasmic bridges. Each cell has two flagella 



Fig. 32. 



Hermaphrodite Colony of Volvox. Covering the surface are hundreds of 

 bi-flagellate units (i) and (5); 2, an egg-cell; 3, a ball of male elements 

 or spermatozoa; 4, a fertilised egg-cell segmenting to form a ball of cells, 

 a daughter colony. After Klein. 



and a directive eye-spot, and the colony swims in a spiral as if it 

 were a single cell. An aggregate has become an integrate. 



Animals that feed easily and abundantly on micro-organisms or 

 on organic debris in the water tend to have more income than ex- 

 penditure; and this naturally leads to growth. When the nutritional 

 superfluity is intermittent rather than constant, the formation of 

 new individuals by budding is more likely to occur than the great 

 enlargement of a single individual. Moreover, the formation of a 

 colony by budding opens up the possibility of arborescence, a very 

 profitable mode of growth, which finds its climax in some of the 

 immense "sea-fans" and "sea-pens", where thousands of individuals 

 live in physical continuity, yet without undue crowding. A third 

 advantage of colony-forming is seen when there is division of labour 



VOL. I I 



