176 



THE EVOLUTION THEORY 



ami salts, which arc of vahic especially in times of drought. Per- 

 haps there is some connexion between this and the fact that limes 

 wither and lose their leaves so qnickly during great summer- 

 heat; these and many other of our trees possess no root-fungi or 



mycorhiza\ 



It is easy to understand, therefore, that genuine ' symbiosis ' may 

 have arisen from parasitism. But that this is not the only path that 

 leads to symbiosis is shown by the cases of animal symbiosis we have 

 already discussed. 



The partnership between polyps and hermit-crabs may have 

 arisen from a one-sided commensalism, since polyps establishing 

 themselves on mollusc shells which were often made use of by 

 hermit-crabs would be better fed than those which settled down on 





Fig. 39. A, fragment of a Silver Poplar root, with an envelope of sym- 

 biotic fungoid filaments (mycelium) ; after Kerner. B, apex of a Beech root, 

 ■with the closely enveloping mantle of mycelium ; enlarged 480 times. 



stones. There are still species which make use of both modes of 

 settlement. Then followed the adaptation of the crustacean to the 

 polyp, for, first, those hermit-crabs would thrive best which tolerated 

 the presence of the polyp ; then those which sought its presence, that 

 is to say, which gave a preference to shells covered with polyps ; and, 

 finally, those which would take no others, and even themselves fixed 

 the sea- anemone upon it, if it chanced to be removed. Intelligence 

 need not be taken into account in the matter at all, not even in the 

 hermit-crab's case. We have only to recall the complex instincts, 

 exercised only once in a lifetime, which compel the silk-worm 

 and the emperor moth to elaborate their effective cocoons. The 

 elaboration of the spinning-instinct can only be due to natural 

 selection, for the insect can have had no idea of the utility of its 

 performance, and the same is true in the case of the sea-anemones or 



