ECOLOGICAL 139 



bearer. Instances need not be multiplied ; it is clear that parasitism, 

 being a one-sided nutritive relation, can be distinguished from 

 shelter-associations. These may indeed develop conversely towards 

 symbiotic partnership, as in the case of the Algaj and fungi which 



rombine into Lichens, in various ways the most successful organic 



)artnership reached in Nature. 



>ARASITES, COMMENSALS, AND SYMBIONS.— From com- 

 mensaUsm, defined as a beneficial external partnership between two 

 organisms of different kinds, ectoparasitism is distinguished by 

 being one-sided, being more or less prejudicial to the host. From 

 symbiosis, defined as a mutually beneficial internal partnership 

 between two organisms of different kinds, endoparasitism is also 

 distinguished by its one-sidedness. As the definitions here adopted 

 are historically justified and convenient, it seems undesirable to use 

 "symbiosis" loosely for the intimate living together of two kinds 

 of organisms, and then to subdivide it into parasitism and com- 

 mensalism, as is proposed by Coulter, Barnes, and Cowles in their 

 excellent Textbook of Botany, 191 1. On this usage a mutually 

 beneficial nutritive partnership, e.g. between clover and its tubercle- 

 forming bacteria, is called "reciprocal parasitism", and "commen- 

 salism" is used to include "those cases of symbiosis in which two or 

 more organisms live together with possible benefit to some or all of 

 the symbions, but with injury to none". Without denying the logic 

 of this treatment, we think that it is more naturalistic, more 

 serial and more convenient, to abide by the ordinary usage, i.e. 

 to distinguish endoparasitism from symbiosis, and ectoparasitism 

 from commensalism. 



But emphasis must also be laid on the fact that parasitism is a 

 nutritive relation. The ecologists just referred to emphasise this, as 

 in distinguishing among plants {a) the independent "autophytes", 

 which obtain all their food from inorganic sources, and (h) the 

 dependent "heterophytes", "whose existence depends upon ante- 

 cedent or coexistent organic forms, because they derive at least a 

 part of their food from organic sources". These heterophytes they 

 then divide into saprophytes, which obtain food from dead organic 

 matter, and parasites, which obtain food or food materials from 

 living organisms. In this classification of nutritive habits, a special 

 corner is needed for carnivorous plants, which obtain their food 

 partly from inorganic sources and partly from the animals they 

 capture. Among the bacteria and other plants that live in the ali- 

 mentary tract of animals, it is sometimes difficult to draw a clear 

 fine between those that live on non-living material and those that 

 attack living tissue. 



STRICTER ANALYSIS OF PARASITISM.— The biological con- 

 cept of parasitism is too often blurred by uncritical usage. When 



