168 THE EVOLUTION THEORY 



ideas, if I may use the expression, must rather have been little fishes 

 and unexpected food — had no cause to shoot its microscopic arrows 

 at it, and did not do so even when the fish concealed itself among 

 the tentacles. This latter habit on the part of the fish would 

 be developed into an instinct through natural selection, since the 

 individuals that most frequently exhibited it would be the best 

 protected, and therefore, on an average, the most likely to survive. 

 Whether the benevolent attitude of the anemone towards the fish is 

 to be regarded as the expression of an instinct is open to dispute, for , 

 it is quite conceivable that each individual sea-anemone is disposed to 

 gentleness by the behaviour of the fish, and so the development of 

 a special hereditary instinct was unnecessary, because without it each 

 anemone reacted in the manner most likely to secure its own 

 advantage 1 . 



The same may be true of the fish as far as laying its booty in 

 the mouth of the anemone is concerned ; there may be no inherited 

 instinct in this ; it may be an intelligent action, which is learnt anew 

 in the lifetime of each individual. 



It might of course be objected to this interpretation that the 

 beginning of the process 3 namely, the assumption that chance frag- 

 ments from the food of the fish falling just on the anemone is very 

 improbable : but I once observed that flat rocks washed over by the 

 sea on the Mediterranean coast (not far from Ajaccio) were so thickly 

 covered with green anemones that at first I took the green growth for 

 some strange sea-grass new to me until I had pulled up a little tuft 

 of the supposed plants and identified them as the soft tentacles of 

 Anthea cereus. Anemones must be equally abundant in the tropical 

 seas of Java, and a sinking; fragment must often alight on the mouth 

 of one of them. 



Much attention and keen discussion have in the last few decades 

 been focussed on cases of symbiosis between unicellular Algae and 

 simple animals. A good example is our green fresh- water polyp, 

 Hydra viridis (Fig. 35, A). Its beautiful colour is due to chlorophyll, 

 and it was long a matter of surprise that animals should produce 



1 Since the above was written Plate lias observed several similar cases in the Red 

 Sea. A little fish lives along with the anemone, Crambactis entrant iaca, a foot in size, and 

 not only conceals itself among its tentacles, but remains among them when the anemone 

 draws them in. These fishes, therefore, must be immune against the stinging-cells of 

 the sea-anemone ; and in the same way another species of fish appears to be immune 

 from the strong poison secreted by sea-urchins of the genus Diadema from the points 

 of their spines, among which the fishes live. This relation certainly seems more like 

 a one-sided adaptation on the part of the fishes than a true symbiosis, but in the cases 

 observed by Sluiter the return service of the fishes seems to be regularly rendered. 

 Here, as everywhere else in nature, there are transition stages, and a one-sid<d 

 protective relation may gradually, under favourable circumstances, be transformed 

 into a symbiosis. 



