202 III. THE EVOLUTION OF VIRUSES 



be perished, followed by the extinction of the parasites themselves. 

 In this way the habit of the host change might be left as an indis- 

 pensable character. 



This may hold also for the plant kingdom. Smuts parasitic on 

 higher plants are known to perform most complicated host change or 

 life cycle ; not only smuts but generally pathogenic fungi are mostly 

 confirmed to be transmitted by insects. Thus insect-fungi relationship 

 is highly organized and has broad biological and evolutional signifi- 

 cance. The insects known to be vectors of plant disease-producing 

 fungi are grasshoppers, crickets, aphids, scale insects, beetles, true 

 bugs, flies, wasps, and bees (115). Plant diseases caused by bacteria 

 are also numerous, and in the dissemination of pathogens from field 

 to field, insects have been recognized as vectors since the days of 

 very early investigations (116). 



Likewise nematodes such as ascaris and filaria belong most pro- 

 bably to the secondary organisms. Ascaris has no insect vector, but 

 its eggs cannot develop unless they are exposed to low temperatures 

 outside the host body, probably contributing to the prevention from 

 its too vigorous proliferation as mentioned above, but also serving to 

 the achievement of the rejuvenation as will be mentioned later. It 

 has been established that certain mosquitoes are vectors of filaria, 

 causing elephantiasis in man. Several other species of nematodes 

 associated with insects possess likewise a heterogenic generation in 

 their life cycle. In addition, each of the three fundamental groups of 

 annelids also gives rise to parasitic species that show, in almost 

 every case, definitely specialized characters of parasitism. 



The arthropods, constituting by far the largest phylum of the 

 animal kingdom, may be the most advanced secondary animals, although 

 the majority of them have already succeeded in getting rid of parasi- 

 tism. Of crustaceans of the arthropods, all stages of parasitism occur 

 especially in copepods. Further, mites and ticks are the parasitic 

 acarina. Nearly all the groups of insects contain species that fail as 

 yet to become free from parasitism either in the larval or in the adult 

 stages. However, only very few insects are totally parasitic in all 

 stages of their life history and these are practically restricted to the 

 true lice and sucking lice. 



It is customarily believed that parasitic organisms have been 

 evolved from free living ones on adapting themselves to parasitism. 

 If this were true, parasitic organisms should have been more advanced 

 than the free living. However, the truth is that the most primitive 

 organisms such as bacteria and protozoa are almost parasitic, whilst 

 the most advanced organisms such as vertebrates including man are 

 entirely free-living without a single exception, presenting a fair con- 



