LECTURE IX 

 ORGANIC PARTNERSHIPS OR SYMBIOSIS 



Hermit-crabs and sea-anemones — Hermit-crabs and hydroid polj'ps — Fishes and 

 sea-anemones — Green fresli-water polyps — Green Amoeba— Sea-anemones and yellow 

 Alga? — Cecropia trees and ants— Lichens — Eoot fungi — Origin of Symbiosis — Nostoc 

 and Azolla aj^parently contradict the origin through natural selection. 



We have already seen, by means of many examples, to what a 

 great degree animals and plants are able to adapt themselves to new 

 conditions of life ; how animals imitate their surroundings in colour 

 and form, how instincts have varied in all directions, how plants have 

 made use of the chance of frequent contact with little animals to 

 obtain nourishment from them, and have developed contrivances 

 adapted for bringing as many of these as possible into their power 

 and causing them to 3deld them the largest possible amount of food. 

 A great many of these could only be interpreted in terms of natural 

 selection, and in others it seemed at least very probable that selection 

 was one of the factors in bring-ino- them about. 



Particularly clear proof of the reality of natural selection is 

 afforded by those cases where one form of life associates itself with 

 a very different one so intimately that they are dependent on one 

 another and cannot live without one another — at least in extreme 

 cases — and that new organs, and, indeed, new dual organisms, are 

 sometimes produced by this interdependence of life. This pheno- 

 menon — so-called 'Symbiosis' — was discovered by two sharp-sighted 

 botanists, Anton de Bary and Schwendener. But Symbiosis occurs 

 not only between plants ; it occurs also between plants and animals 

 and between two species of animal, and we understand by it a life of 

 partnership depending on mutual benefits, so that each of the two 

 species affords some advantage to the other, and makes existence 

 easier for it. In this respect Symbiosis differs from Parasitism, in 

 which one species is simply preyed upon by another without receiving 

 any benefit from it in return, and also from the more innocent 

 Commensalism of Van Beneden, the table-companionship in which 

 one species depends for its existence on the richly-spread table of 

 another. Symbiosis is particularly interesting, because, in addition to 

 extreme cases with marked adaptations, many occur which are of 



