BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



255 



two factors are characteristic of the be- 

 havior of both parasite and host. Both show 

 more or less active oflFensive and also de- 

 fensive activities. Active injuries and more 

 passive resistances are often involved for 

 both the invading and the invaded organ- 

 isms. 



The wide variety of parasite-host rela- 

 tions are related to (1) differences in both 

 host and parasite species, populations, and 

 individuals, resulting from diverse influ- 

 ences both hereditary and environmental; 

 (2) differences resulting from the degree of 

 adaptation to the given parasitism, includ- 

 ing (3) differential responses to aberrant or 

 unusual invasions. 



The various anti-alien reactions of both 

 parasite and host are much influenced by 

 immediate reciprocal stimulation and follow 

 no generally uniform order. The aggressive 

 anti-invasion activities of the cells and tis- 

 sues of the host are derived from his normal 

 physiology and are, in general, lytic (i.e., 

 dissolving). Among the primary difficulties 

 that affect the parasite-host relation are the 

 serological differences between animals of 

 all kinds. Similarly, the antihost activities of 

 the parasite are related to those in its ances- 

 tral, free-living conditions. Until changed in 

 evolution, these effects tend strongly toward 

 being essentially toxic or otherwise destruc- 

 tive. 



The self-protective activities of host and 

 parasite similarly include both preadapta- 

 tion and adjustments evolved in the course 

 of evolution of the parasitic relation. The 

 fairly successful, but still vulnerable, evolv- 

 ing endoparasite has achieved heightened 

 surface resistance, or a new covering sub- 

 stance hke that furnished by bacterial cap- 

 sules, or some degree of immunity to the 

 antigens of the host. The fairly resistant, but 

 still vulnerable, host has met the situation 

 by the formation of a series of specific anti- 

 bodies and the development of phagocytic 

 cells devoted to either internment or de- 

 stniction of the invading population. 



The development of a more or less bal- 

 anced condition between host and patho- 

 genetic (or potentially pathogenic) micro- 

 organism requires three conditions: pro- 

 longed association, opportunity for a hic;h 

 proportion of the host species to become in- 

 fected, and the absence of any important 

 means by which the pathogen can survive 

 for long periods in the absence of the host. 

 When such conditions prevail, a low grade. 



widespread infection should exist vdth httle 

 host mortality. 



A virus or a bacterial population may 

 be introduced into a new host in which it 

 can flourish with or without killing the host, 

 and be transferred from one individual to 

 another in the newly infected species. If the 

 new host is relatively solitary, the infection 

 may kill an individual or two and go no 

 further; if the new host is gregarious, a 

 new ecological conflict is set going that will 

 eventually be resolved by the elimination 

 of the virus, the death of the infected host 

 population, or the development of a new 

 balance (Burnet 1945). 



The parasitic habit has multiple origins. 

 It can develop from monophagy as well as 

 from commensalism, or from mutualism. It 

 may evolve toward mutualism, but parasites 

 are usually too specialized to be able to 

 move away from the parasitic adjustment. 

 We would expect that external commensals 

 and epicoles (epibionts) of various kinds 

 could become ectoparasites with relative 

 ease. A hint as to how free-livins: forms may 

 become epicoles is shown by the observa- 

 tion of Mashtaler (1937): Wlien Htidra 

 was placed with one of its predators, Lim- 

 naea staenalh. only those hydra survived 

 that happened to fasten themselves to the 

 snail's shell. Internal commensals or mutu- 

 alists could become endoparasites. Certain 

 nematodes, among others, may have begun 

 as saprophytes. Some free living nematodes 

 take only liquid food. The narrow lumen of 

 the gut of Rhabditis will admit only solid 

 particles of the size of bacteria, and the 

 food taken consists of material liquefied by 

 bacterial action. It is a short step to the in- 

 gestion of food liquefied by the digestive 

 ferments of a living host, as happens \vith 

 the nematode Ascaris. Flesh flies can trans- 

 fer from laying eggs on dead animals to lay- 

 ing them on decaying flesh still attached to 

 the wound of a living animal; it is then an 

 easv step to laying eggs on the "clean" 

 flesh of an open wound. 



Sessile animals are already partially pre- 

 adapted to parasitism by adaptations for 

 attachment, for example, as are many small 

 species that are negative to light, positive 

 to touch stimuli, and capable of living in 

 babitats with rediiced oxygen tension. The 

 parasitic habit has something in common 

 with cave dwelling, and some similar pre- 

 adaptations appear to be involved. 



