64 SYMBIOGENES1S 



The ample tribute which it receives suffices for its needs and also for 

 the provision of its eggs. But the weakness of the system here discloses 

 itself. This handing of nitrogen-containing substances to and fro from 

 animal to plant and from plant again to animal cannot go on indefinitely 

 or without loss. Sooner or later, the animal finds itself lacking in 

 essential, nitrogen-containing food-materials. Supply fails to equal the 

 demand. Then the animal is under the dire necessity of digesting its 

 algal cells. To satisfy an imperious present need, the plant-animal 

 destroys the source of its supplies. 



Thus the animal repudiates the association and, having digested its 

 green cells, C. roscoffensis dies of the very complaint nitrogen-hunger 

 which the green cells sought to avoid by their intrusion into the body 

 of the animal. To dismiss the association between animal- and plant- 

 constituent of the plant-animals by labelling it symbiosis is to miss 

 the varying significance of the association. Looking at the relationship 

 from the standpoint of the animal, it is one of obligate parasitism. 

 Apart from their algal cells, C. roscoffensis and C. paradoxa are unable 

 to live. The existence of either species depends upon the infection of 

 the individuals of each successive generation. Where the infecting 

 organism is absent, there C. roscoffensis does not exist. Hence its 

 restricted range. From the standpoint of the species " infecting 

 organism," the relation of certain of its individuals with C. roscoffensis 

 or C. paradoxa is an episode without significance. Unlike the animal, 

 which bears the inherited impi-ess of the relation in its lack of an 

 excretory system and in the habit of patient waiting abiding the 

 question of infecting the alga is free. Of a swarm of flagellated green 

 cells, some small percentage meet the picturesque fate of forming a tissue 

 in the body of an animal. The others pursue a less romantic adventure, 

 either as green, self-supporting organisms or as colourless cells which 

 batten on the offal of the sea. 



If all's well that ends well, we have thus good reason to 

 think that all was not well from the very beginning in the rela- 

 tions between plant and animal as represented by the 

 Convoluted arrangement. 



By hinting at " obligate parasitism," Prof. Keeble implies 

 that some deeper cause than a mere normal nitrogen require- 

 ment is here involved. We can have no " obligate parasitism " 

 without a parasitic diathesis, i.e., an abnormal nitrogen 

 hunger. This abnormal craving is bound to increase, and can 

 be satisfied the less, the greater the divorce from genuinely 

 symbiotic relations becomes. That the same morbidity in some 



