SOCIAL LIFE 295 



Social Impulses and Observances. 



Social conditions can be observed to occur among the lower 

 animals (medusae, molluscs, insects). It seems natural to con- 

 clude when one sees the individuals of these orders forming 

 compact congregations that if they are not actuated by in- 

 dividual reciprocal affection there is some unconscious bond 

 uniting the species. In many instances birth-affinities may 

 have conduced to this community-life. No gain whatever to 

 the individual results from this form of socialism ; mutualism, 

 on the other hand, is always associated with some advantage 

 to one or both the contracting parties, who may or may not be 

 physically affected by their mutual relationship. The relation 

 of the peacock to the tiger and the hen curassow to the puma 

 is an offensive and defensive alliance. The strong protects 

 the weak in consideration of the latter's usefulness in warning 

 hi m of danger. This is the case with the shark and the pilot 

 fish ; in return for definite protection the pilot fish reconnoitres 

 for the shark. What is the alliance between the hermit crab 

 and the sea anemone on his shell but a mutual compact 

 that the crab will protect the actinia with his claws in return 

 for the protection of the other's stinging powers ? Still it is 

 somewhat inexplicable why the ants, which are otherwise so 

 pugnacious, should tolerate in their nests a large number of 

 larval, or mature, chafers (Scarabaeidae) ; possibly the ants are 

 fond of some excretion or exudation of these myrmecophilae ; 

 for it is inconceivable to suppose the ant acts disinterestedly. 

 It is quite easy to explain the friendship between the starling 

 and the sheep, between the maggot-pecker and the great African 

 mammals ; both parties confide in one another because the 

 advantage is mutual, the insectivorous birds finding an abund- 

 ance of food on the skin of the animal who is relieved of his 

 parasites by the extraordinary vigilance of the birds. 



A sort of travelling companionship is set up when one 

 species uses the strength of another as a vehicle, e.g., the turtle 

 and the sea-urchin (Echinus) clinging to it, or the aplysia riding 

 on the larger crabs. This travelling at some one else's ex- 

 pense may be regarded as a sort of parasitism ; it may extend 

 further to a table companionship (commensalism), when one 



