PARASITISM AND SYMBIOSIS — CAUULERY. 401 



But even if this explanation would fit certain cases, as might be 

 suggested by present experimental research on heredity, we hesitate 

 to admit its general applicability, in view of the mass of adaptive 

 arrangements which exist in nature, and especially in view of the 

 graduated series presented by these arrangements. Can a harmony 

 so exactly coordinated with the environment exist unless the environ- 

 ment has had something to do with bringing it about? Can such a 

 general fact be due to a mere series of coincidences between environ- 

 ment on the one hand and on the other the special constitution of 

 organisms as well as the necessary laws of their transformations; 

 setting aside naturally all teleological and creationistic ideas ? 



We hesitate still more, because we see that the reaction of the 

 individual to the environment is in large measure adaptive ; the trans- 

 formations of the plant passing from the plain to the mountain, or, 

 inversely, are proof of it. But these • modifications do not show 

 themselves to be hereditary. Perhaps the solution of the difficulty 

 lies in the general indications which paleontology give us. The 

 different types have not varied in time in a uniform and permanent 

 manner. Each seems to have had its period of variability. At this 

 period were the hereditary variations really independent of the 

 environment, as we see them to-day in organisms which are per- 

 haps in a phase of stability, or, on the contrary, was individual 

 adaptive reaction inherited? 



However this may be, the problem of adaptation remains funda- 

 mental. Parasitism offers an excellent field for its study, while at 

 the same time it raises the whole question of evolution in all its di- 

 versity. Moreover, its aspect is not merely morphological; it is at 

 the same time a problem of a physiological order. 



From the physiological point of view, parasitism raises a series 

 of questions which have the greatest interest to general biology. 



In the first place, it is an excellent field for the study, under par- 

 ticularly well-defined conditions, of the reciprocal influences of or- 

 ganisms. All organisms are dependent on each other. Among them 

 there exists a vital competition in the broad sense, a phenomenon 

 of primary importance the demonstration of which is one of Darwin's 

 greatest titles to renown. But here the bond between parasite and 

 host is a definite one, and the reciprocal action of the two antago- 

 nists is clearly limited. A definition of parasitism is again needed. 

 An organism is a parasite of another when it lives directly at the 

 expense of that other, feeding on its substance or utilizing the activi- 

 ties of the other's organs to obtain its own subsistence and accom- 

 plish its life cycle. Except the plants, which directly assimilate the 

 carbon of the atmosphere or the nitrogen and the mineral constitu- 

 ents of the soil, all organisms subsist at the expense of others, and 

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