ECOLOGICAL 141 



PARASITISM AND PREDATION.— The concept of true para- 

 sitism would be clearer if there were a separate sub-concept for 

 those cases where the infesting organism lives an energetic predatory 

 life; and is thus not so much a parasite, as a veritable beast of prey 

 devouring its host from within, or, in the case of some ectoparasites, 

 from without. From a broad physiological outlook, parasitism is a 

 negative reaction to the struggle for existence; it implies the dis- 

 covery and adoption of a mode of life along the line of least resistance. 

 On this view the diagnostic feature of true parasitism is its very 

 evasion of strenuous struggle and individual endeavour. To put it 

 more metaphorically, the swimmer becomes a drifter. Yet the 

 larvae of an Ichneumon-fiy, which are hatched out within a cater- 

 pillar and proceed to devour it from within, are hardly less predatory 

 than the lion which devours the antelope from without. These Ich- 

 neumon grubs have none of the degenerative stigmata of thorough- 

 going parasites. Similarly, while many of the Protozoon parasites, 

 like Gregarines, are sluggish throughout a great part of their life, 

 and may even remain for a long time within the same cell, it is more 

 difficult to apply the strict term parasite to such organisms as the 

 exceedingly active Trypanosomes that cause vSleeping Sickness and 

 allied diseases. Thej^ have their quiescent phases, no doubt, but 

 much of their life is spent in charging about among the blood 

 corpuscles at high velocity, though undeniably entering them at 

 last. Many of the Protozoa — especially in the entirely parasitic 

 Gregarines and other Sporozoa— are much simplified cells, but that 

 cannot be said of the Trypanosomes, which are highly specialised 

 flagellate Infusorians, and might well be ranked as unicellular 

 predatory animals that work destruction from within, and this 

 especially in an unwonted host, like man or horse. 



And agahi, as regards ectoparasites, is not the definition some- 

 what blurred by including such types as the flea? That it is more 

 habitual than a leech in its blood-sucking does not make it less 

 predatory. Its compressed body may be adaptive to escaping capture 

 as it moves swiftly among the hairs of a mammal's skin, but it has 

 no marks of degeneracy in its adult life. It is on an ecological level 

 quite different from that occupied by such types as the mange- 

 mites of the dog. Our point is that the concept of parasitism becomes 

 more useful when habitually predatory types, external or even 

 internal, are placed at its very begmnings, if not positively excluded. 



CLASSIFICATION OF PARASITIC ANIMALS. — Protozoon 

 parasites are illustrated {a) by the entire class of Sporozoa, including 

 among its half-dozen orders, not only the Gregarines, but types like 

 the Malaria organism (Plasmodium) ; [b) by some Rhizopods, such 

 as the Amoebae of man's intestine in dysentery, and in his gums 

 in pyorrhea; and (c) by some Infusorians, such as the mouthless, 



