126 ANIMAL INDIVIDUALITY [CH. 



systems," other individualities. Out of every little 

 accidental company she tends to make an inter-related 

 whole whose parts are largely dependent on each other, 

 and only slightly on other wholes or their parts. 



It will be necessary to give a few more examples 

 of the inter-relation of two distinct species before 

 developing this idea. A very instructive example is 

 that of Convoluta roscqffensis, a marine flatworm. 

 Its story has been so clearly told by Prof. Keeble (10) 

 that here an outline will be enough. In nature, the 

 worm is always associated with a unicellular green 

 plant which lives in great numbers beneath its skin. 

 The plant on the other hand is found abundantly 

 apart from the worm, but swarms round the egg- 

 capsules in order to procure nitrogenous food, and 

 gets ingested by the young animal. Unless this 

 happens, the worm cannot develop further the 

 presence of the green cells is the only stimulus which 

 will start its machinery on the next stage of its 

 working. At first both members gain from the 

 association, much as in the lichen, but finally, after 

 the worm (which at last takes no food, but depends 

 entirely on the surplusage of the alga) has produced 

 its eggs, it finds itself short of nitrogenous material, 

 and begins attacking and digesting the green cells ; 

 they cannot last for ever, and when they are all gone 

 last of all the worm dies also, trusting to chance that 

 its young will find new algae. This shows a transition 



