ORGANIC PARTNERSHIPS OR SYMBIOSIS 171 



animal digestive juices; they must, therefore, have undergone some 

 variation, and adapted themselves to the new situation. Probably 

 their cell-membrane has become impenetrable to the stuffs which 

 would naturally digest them, an adaptation which could not be 

 referred to direct effect or to use, but only to the accumulation of 

 useful variations which cropped up — in other words, to natural selec- 

 tion. That any adaptive variation has taken place on the part of the 

 host, whether polyp, amoeba, or Infusorian, cannot be made out. None 

 of these have altered their original mode of life ; they do not depend 

 on the nourishment afforded by the algae, but feed on other animals, 

 if these come in their way, and they live in water rich in oxygen like 

 other species allied to them, and therefore are not altogether de- 

 pendent on the algae in this connexion : but they can no more help 

 having their partners than the pig can help having Trichinae in its 

 muscles. 



Similar plant-cells, not green however, but yellow, called zooxan- 

 thellae, live in great numbers in the endoderm of various sea-anemones 

 and in the soft plasmic substance of many Radiolarians. In both these 

 cases we must look for the benefit they confer on their host in the 

 oxygen they give off, for, like the green zoochlorellaa, they break up 

 carbonic acid gas in the light, and give off oxygen ; they no longer 

 occur, as far as is known, in a free state, but are always associated 

 with the host, and they must therefore have altered in constitution, 

 and have adapted themselves to the conditions of the symbiosis. 



Higher plants, too, sometimes have s}mibiotic relations with 

 animals ; the most remarkable and best-known example is the relation 

 between ants and certain trees, in which the ants protect trees which 

 afford them in return both a dwelling-place and food. We owe our 

 knowledge of these cases to Thomas Belt and Fritz Muller, and more 

 recently it has been materially increased by Schimper's researches. 



In the forests of South America there grow ' Imbauba,' or can- 

 delabra-trees, species of the genus Cecropia, which well deserve their 

 name, for their bare branches stretch out like candelabra, and bear 

 little bunches of leaves only at their tips. These leaves are menaced 

 by the leaf-cutting ants of the genus (Ecodoma, which attack 

 numerous species of plants in these regions, often in tens of thousands, 

 biting off the leaves, cutting them in pieces on the ground, and carry- 

 ing them on their backs piece by piece to their nests. There they use 

 them to make a kind of compost heap, on which fungi, to which the ants 

 are very partial, readily grow. The candelabra-tree protects itself 

 from these dangerous robbers, inasmuch as it has established an 

 association with another ant (Azteca instabilis), which finds a safe 

 I. M 



