328 Relations within the Species 



the group. In some sessile colonial animals, such as certain of the 

 coelenterates, different individuals are specialized for definite func- 

 tions. This differentiation of zooids pemiitting a division of labor 

 within the colony reaches its highest development in siphonophores 

 such as the Portuguese man-of-war. Different zooids of this animal 

 are specialized for flotation, protection, nutrition, or reproduction. 

 In some siphonophores zooids are also specialized for locomotion and 

 produce a movement of the whole colony by rhythmic ejection of 

 water from their cavities. Certain colonial Protozoa and Protophyta 

 also show a division of labor. Although this development among 

 unicellular colonial forms is very primitive, its advantages may have 

 led in the past to the evolution of multicellular organisms. 



Among organisms that form groups but in which the individuals re- 

 main separate we find other types of division of labor. The most 

 fundamental and widespread of these is the differentiation of the 

 species into two sexes. This division of labor has arisen in both the 

 animal and plant kingdom, and interestingly enough the appearance 

 of sex occurred long after plants and animals had evolved from their 

 common ancestral unicellular form. We have here a remarkable case 

 of parallel evolution in which the same adaptation arose indepen- 

 dently on at least two occasions. 



A variation in activity among members of a group without any 

 special morphological adaptation is seen in the phenomenon of social 

 dominance. This is the establishment within the group of a social 

 hierarchy in which an animal of higher position outfaces or drives 

 away an animal of lower position ( Allee, 1951 ) . Such "peck orders" 

 were first recognized in flocks of fowl, but now dominance-subordina- 

 tion relations are known to exist among certain groups of fishes, 

 lizards, rodents, ungulates, carnivores, and other animals (Collias, 

 1944, 1952). The peck order among hens establishes the social posi- 

 tion of the birds in the flock. A dominant hen has attained the right 

 to peck a subordinate hen without being pecked back. When the 

 social organization of a flock has become established, the group func- 

 tions more smoothly and less fighting occurs, since protocol is recog- 

 nized by all members for all group activities. 



Another type of division of labor in a group is leadership. Within 

 a flock of goats one animal will become recognized as the leader in the 

 wanderings of the group; but this animal is not necessarily the domi- 

 nant individual in the sense of the term used above. The existence 

 of a definite leadership was also demonstrated in a band of monkeys 

 inhabiting a tropical island. One individual always led the way when 

 the band moved through the aerial pathways of the treetops. When 



