254 



ANALYSIS OF THE ENVIRONMENT 



merits in which philosophers indulge. Since 

 philosophers often contend that science is 

 not concerned with values, it is a matter of 

 some interest that these value determina- 

 tions or judgments are an integral part of 

 ecology. It may be that evolutionary survi- 

 val values with real or implied objectivity 

 should be separated rather sharply from 

 subjective value judgments such as wiU be 

 illustrated in the following paragraph. 



The whole parasitic habit is regarded 

 with repugnance by most people, including 

 many biologists. They appear to make in- 

 formal emotional value judgments to the 

 eflFect that external predation of a relatively 

 large predator on small prey, as of fox or 

 marsh hawk on meadow mice, is praise- 

 worthy and that, in contrast, internal pre- 

 dation of hookworms on the gut wall of 

 their host is ignoble. An ecological approach 

 is necessarily wholly objective. A pallid en- 

 doparasite is rarely a thing of visual beauty, 

 yet the intricacy and delicacy of its adjust- 

 ments to life in a Hving environment— often 

 a complex series of such adjustments— de- 

 light the initiated student; harmony is the 

 essence of beauty. It must be admitted that 

 the individual parasite, embodying the re- 

 sults of regressive evolution, may be a less 

 obviously harmonious organism than its 

 free-living ancestor. Our subjective judg- 

 ments are evidently based in part on the 

 obvious harm to ourselves and our commen- 

 sal domestic animals from the more destruc- 

 tive parasites that attack them and man. 

 Some aspects of the remarkable range of the 

 natural history of parasitic adjustments are 

 outlined by Pearse (1939). 



Living organisms, as hosts to parasites, 

 form one of the three major habitats on the 

 earth, comparable to the aquatic and ter- 

 restrial habitats in which the hosts them- 

 selves dwell. This living habitat presents 

 various niches, many of which are occupied 

 by assemblages of organisms comparable 

 vvith those of shore lines, abyssal depths, 

 caves, forests, or streams, except on a scale 

 necessarily smaller in actual space, though 

 not necessarily smaller in terms of popula- 

 tion numbers. The body surface is a gener- 

 alized habitat with habitat niches provided 

 by hair follicles, pores, glands, and the var- 

 ious body recesses that have external open- 

 ings. The alimentary canal is a particularly 

 well-inhabited niche. Air passages, coelom, 

 muscles, internal glands, central nervous 

 system, for example, each may support a 



varied ecological assemblage of plants and 

 animals (cf. Smith, 1934). 



A habitat that is itself alive offers ready 

 food for those equipped to take it, provided 

 the associated physical and chemical con- 

 ditions can be tolerated. There are internal 

 habitat niches in the bodies of animals that 

 are regions of reduced oxygen tension, in- 

 habited by successful endoparasites able to 

 carry on oxidations wholly or in part by 

 anaerobic metaboUsm. They must also be 

 immune to diverse protective mechanisms 

 available to the host, including wandering 

 macrophages, specific antibodies, and coun- 

 termeasures that depend on individual or 

 cooperative behavior adjustments. The abil- 

 ity of the host to alter its habits and habi- 

 tat may be an important part of its total 

 protective power. The ecological relations of 

 host and parasite, and of both with their 

 biotic and physical environments, are varied 

 and intimate. One is continually reminded 

 that it is a short step from considering the 

 biotic aspects of the environment to deal- 

 ing with food webs and other phases of the 

 ecological community in all its complexity. 

 Parasites must manage to retain position, 

 often in opposition to currents and other 

 forces of considerable strength. Particularly, 

 parasites must be able to transport them- 

 selves, or secure suitable transport of 

 enough representatives, to insure the con- 

 tinuing existence of populations of parasites 

 in habitats anyone of which at best has only 

 a temporary existence. Except in passive 

 transport, in which a predator eats its prey, 

 parasites and all, and thereby gains an in- 

 fection, the parasite population periodically 

 is exposed to the rigors of the nonliving 

 environment. Such exposure is often fatal, 

 since in their active stages even the regular 

 dissemules may tolerate only a restricted 

 range of environmental conditions. Encysted 

 stages of parasites, however, may be resist- 

 ant to environmental conditions. Encysted 

 nematodes have been found alive after 

 twenty years' encystment, and this is by no 

 means the longest case on record. 



The understanding of ecological aspects 

 of parasite-host interrelations requires a 

 knowledge of the physiological needs and 

 interactions of both populations. Such infor- 

 mation is hard to obtain even for a given 

 parasite in a particular host. Theobald 

 Smith (1934), a distinguished pioneer in 

 this field (p. 29), in his search for under- 

 lying relations presented the hypothesis that 



