PARASITISM 



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phase in mosquitoes. So also with filariasis ; the 

 filariae in man, their ova, and their embryo-worms, 

 are one phase of filariasis ; and the embryo-worms 

 in certain structures of the mosquito are another 

 phase. The Plasmodium malarice and the filaria 

 are instances of a law of animal life that holds 

 good also of plant life : — 



"All plants and animals possess parasites, and 

 thousands of different species of parasites have been 

 closely studied by science ; we therefore know much 

 about their general ways of life. As a rule, a 

 particular species of parasite can live only in the 

 particular species of animal in which, by the 

 evolution of ages, it has acquired the power of 

 living. It is therefore not enough for the parasites 

 of an individual animal — say a man— to be able to 

 multiply within that individual, but they must also 

 make arrangements, so to speak, for their progeny 

 to enter into and infect other individuals of the same 

 species. They cannot live for ever in one indi- 

 vidual ; they must spread in some way or other to 

 other individuals. 



"The shifts made by parasites to meet this 

 requirement of their nature are many and various, 

 and constitute one of the wonders of nature. Some 

 scatter their spores and eggs broadcast in the soil, 

 water, or air, as it were in the hope that some of 

 them will alight by accident on a plant or animal 

 suitable for their future growth. Many parasites 

 employ, in various ways, a second species of animal 

 as a go-between. Thus, some tape-worms, and the 

 worms which cause trichinosis, spend a part of their 

 lives in the flesh of swine, and transfer themselves 

 to human beings when the latter eat this flesh. 



