SYMBIOSIS 35 



In spring the wingless female, without the intervention of a male, 

 starts at a terrific pace to propagate the species by a process which is 

 known popularly as budding, and scientifically as parthenogenesis. It 

 throws off living young in reckless profusion, and continues to do so 

 for months. The young are soon able, also without the intervention of 

 a male, to propagate in the same manner, as are the generations that 

 follow in rapid succession, and in consequence the descendants of a 

 single female would, but for the checks imposed by Nature and by man, 

 in the course of a season cover an enormous area. As the summer 

 advances, however, the rose-aphis, like Convoluta roscoffensis previously 

 mentioned, feels the urgent need of a change of diet, and, unlike that 

 animal, is able to get it by going farther afield, for it has for a brief 

 period entered the pupal state and has emerged with wings. So it 

 deserts the rose, on which it has hitherto lived, and flies to certain 

 grasses, feeding on them for some weeks, and finally, after mating, 

 returning to the rose, where it commences a wholly different method 

 of propagation by means of eggs. When we remember the effect that 

 a change of food has on bees, converting an ordinary worker or 

 undeveloped female into a queen capable of egg-laying, it is a fail- 

 inference that this is also the cause or at least one of the causes of 

 the remarkable change in the structure and habits of the rose-aphis. In 

 the brief sketch of the life-history of C. roscoffensis, we saw a similar 

 sequence. So long as the animal was content with its usual food it 

 merely continued to grow, but when it turned to other fare and devoured 

 the cells within its body, it began to produce eggs, "the substance of 

 which," as Prof. Keeble tells us, "was supplied by those cells." 



What emerges is this : 



Different modes of nutrition are responsible for different 

 modes of propagation and for different forms and structures. 

 This fact, as we have seen, is ultimately connected with bio- 

 economic differences, i.e., with differences of work. In the 

 last analysis permanence and survival in the organic world 

 depends on work as instanced by the value and indispensa- 

 bility of photosynthesis. Whether a particular organism be 

 it plant or animal is to be stable and permanent, similarly 

 depends on its continuance of work. Where depredation, sur- 

 feit, and parasitism take the place of work, the resulting 

 organisation must become unstable. What is it, then, that 

 constitutes "usual food" for an animal? That food, it must 

 be answered (from the point of view of value and of per- 

 manence), which is obtained by work, i.e., as a result of 



