THE FACTORS OF EVOLUTION 181 



in which the strongest, quickest, and most cunning survived, 

 only to be compelled to fight again on the morrow. No need 

 for the spectators to ' turn down the thumb,' for in this battle no 

 pardon was given. 



Kessler, and recently Kropotkin in his work on ' Mutual Aid 

 in Evolution,' have shown that a brutal fight for existence is 

 not the only factor in evolution, for in addition to strife and 

 rivalry we find numerous cases of alliance and mutual support- 

 This friendly living together has undoubtedly had a far-reaching 

 influence upon the development of physical characteristics as 

 well as upon the improvement of the mental qualities. Time for- 

 bids my dealing in detail with animal colonies and communities, 

 but I will mention a few instances of symbiosis, because they 

 produce sometimes almost the same kind of change which we 

 saw as having taken place in consequence of the domestication 

 of animals. When several years ago I was at the zoological 

 station at Naples I derived much pleasure from spending a 

 considerable part of my leisure time in the magnificent aquarium. 

 The inhabitants of one of the tanks consisted of several individuals 

 of the well-known hermit-crab (Pagurus calidus), and of some 

 Actiniae, the beautiful 'sea-roses' or ' sea-anemones.' They are 

 children of warm climes, and are found in large swarms on the 

 ocean-bed, attached to rocks or empty shells, waiting with 

 spreading tentacles for their prey. Near Capri one may see with 

 each wave many thousands of brilliant-red ' sea-anemones ' which 

 surround the entire coast of the island as with a girdle of fire. 

 But their magnificent and alluring exterior is protected by a large 

 number of offensive elements from which fine lassoes, bathed in 

 poison, are thrown out on the least provocation, numbing or 

 killing both prey and enemies, and causing a violent burning 

 pain. Other tentacles are concealed in the body of the sea- 

 anemone, which, in case of need, are thrown forward through 

 the mouth aperture. On account of these poison-lassoes the 

 Actiniae are greatly feared by all animals, while there are hardly 

 any of which they themselves need be afraid. 



The hermit-crab is a near relative of our crayfish and of 

 about the same size. It owes its name to its mode of life, for 

 it has assumed the peculiar habit of making its domicile in the 

 shell of snails, the legal owner of which it effectively disposed of 



