jc() PARASITISM 



from beef pancreas that it has been suggested that it is not formed 

 by the worm, but that it is produced by the host under stimulation. 

 However that may be, the production of anti-enzymes is not 

 pecuHar to the parasite, and at the most, as was said at the 

 beginning of this discussion, it is only making special use of a 

 general property of living tissue. 



The second aspect of the chemical relationship is that of the 

 reaction of the host to the parasite, which leads, it is to be pre- 

 sumed, to the host-restriction of the parasite. Sometimes only a 

 single species of host seems possible ; thus all attempts to infect 

 chimpanzees with Plasmodium malaricE have failed ; human P. 

 vivax transferred to chimpanzees disappeared and apparently gave 

 no infection, but that it was present was proved because another 

 man could be infected from the ape. At the other extreme adults 

 of Trichinella spiralis have been found naturally in man, pig, 

 fox, cat, and both species of rat, and experimentally all mammals 

 that have been tried have taken the infection, and although 

 birds are not susceptible, salamanders are if they are kept at 30° C. 

 Some degree of restriction between these extremes is the common 

 state. It is often found that, while transference from one host 

 to another is impossible, parasites of morphologically identical 

 form occur in the second host. Thus the three species of Plasmodium 

 found in man are all paralleled in the higher apes ; Entamoeba 

 histolytica of man and E. ranarum of frogs are morphologically 

 indistinguishable, and, except for the size of the labial teeth, the 

 same is probably true of Ascaris lumbricoides and A. suillce. 

 Whether such forms should be classed as separate species is a 

 question beyond the scope of this chapter, but it is well to remem- 

 ber that man, living in a visual world, is apt to overvalue visual 

 stimuli. He cannot consciously distinguish E. histolytica and E. 

 ranarum, because he tries to do so by his eyes alone ; the epithelium 

 of his intestine can, however, tell the difference between them. 

 Other animals can make a similar distinction between the morpho- 

 logically identical Trypanosoma equiperdum living in the con- 

 nective tissue of horses and transmitted during coitus, and T. 

 evansi living in blood and carried by a tabanid vector. We may 

 safely assume that the differences between all these forms, 

 whether we call them biological races or distinct species, are 

 fundamentally chemical. Such biological races, though commoner 

 amongst parasites, are not confined to them, being also found, for 

 '■^^.tance, amongst phytophagous insects. 



