ORGANIC PARTNERSHIPS OR SYMBIOSIS 167 



them, to ' milk ' them, is also advantageous ; it must be an old 

 acquisition, an instinct early developed, for in several species it has 

 gone so far that the aphides are carried into the ants' nest, and are 

 there (as one might say) kept and tended as domesticated animals. 



A pretty ease of symbiosis between two animals is reported by 

 Sluiter, and I mention it because it concerns a vertebrate animal, 

 and intelligence has something to do with it. In the neighbourhood 

 of Batavia there are frequently to be found on the coral reefs large 

 yellow sea-anemones, with very numerous and comparatively long 

 tentacles, and a little brightly -coloured fish, of the genus TracldcMhys, 

 makes use of these forests, beset with stinging-cells, to find security 

 from its enemies. These appear to be numerous, for in an aquarium, 

 at any rate, the little fish very soon falls a victim to one or other 

 of them, unless he is supplied with the protective sea-anemones. 

 When this is the case it swims blithely about among the tentacles, 

 and the sea-anemone does not sting it ; for there has been a modi- 

 fication of instinct on its part as well as on that of the fish. The 

 advantage it gains from the fish is, that the latter brings large 

 morsels of food — in the aquarium, pieces of meat — into the anemone's 

 mouth. In doing so it tears away fibres for itself, and even if the 

 Actinia has swallowed pieces too quickly, the fish pulls them half out 

 of the gullet again, and only relinquishes them to be consumed by its 

 partner when it has satisfied its own appetite. In this case, again, 

 the modification of the instinct is the only adaptation which has been 

 brought about by the symbiosis, and its origin seems diflicult to 

 understand. How can the fish have first formed the habit of putting 

 its prey into the mouth of the anemone instead of eating it directly ? 

 Although in many cases it is diflicult to guess at the beginnings of 

 a process of selection, because they are scarcely discoverable in 

 the subsequently accumulated variations, yet in this instance we may 

 perhaps picture them to ourselves in this way : The fish was in 

 the habit of letting fall pieces of food which could not be swallowed 

 whole, and of diving down upon them repeatedly, to tear off 

 a fragment each time. As the sea-fioor in flat places is often covered 

 with sea-anemones, these pieces would often sink down upon one, 

 which would welcome it as a dainty, and set about swallowing it 

 slowly in its own fashion. The fish must then have found by 

 experience that it could tear off little bits much more easily from 

 a piece that was held firmly by the anemone than from one that was 

 lying loose upon the ground, and this may have caused it to do 

 intentionally what was at first done by chance. But the sea- 

 anemone, suffering no harm from the fish — indeed, its association of 



