X. THE SECONDARY ORGANISMS 201 



bacteria are mostly parasitic, and all the major groups of protozoa, 

 except the sporozoa and the sustoria, involve both free-living and 

 parasitic forms. Sporoza are entirely parasitic and sustoria are 

 sedantry, although they are often found attached to the surface of 

 fresh-water organisms. Many parasitic protozoa, especiallly sporozoa, 

 undergo extremely complicated life cycles, involving not only several 

 successive hosts but also distinct evolutionary states that are morpho- 

 logically different. 



It is noticeable fact that exceedingly vast number of species are 

 known in protozoa, over 15,000 species having been described, and that 

 most modern authors are inclined to consider many groups of them 

 as so many phyla (114). This would follow naturally if protozoa were 

 evolved from viruses continuously being generated de novo. Parasi- 

 tism must be the most distinct characteristic feature of the secondary 

 organism, but such an existence of a vast number of groups, between 

 which no phylogenetic correlation appears to be present, must likewise 

 be one of their chief characteristics. 



The so-called mesozoa, which was once recognized to be intermediate 

 between protozoa and metazoa, are totally parasitic, and there seems 

 no doubt in their secondary nature. Notwithstanding their highly 

 evolved state, animals belonging to platyhelminthes such as flukes and 

 tapeworms are still sticking to their parasitic nature. These animals 

 are not only parasitic but also still clinging to the habit of virus of 

 changing host and unable to proliferate without the troublesome host 

 change. Thus host change must also be one of the characteristics of 

 the secondary organisms. Malaria-producing protozoa and a kind of 

 mosquito, trypanosomes and a fly, spirochaetes of reccurent fever and 

 a tic or louce, — these are all the examples of the attachment to the 

 habit of host change. ' 



In the case of viruses the host change may be indispensable for 

 the purpose of rejuvenescence, but it is rather strange that the 

 secondary organisms are frequently clinging to the habit still at present 

 when they have acquired much more suitable means, namely sexual re- 

 production, for the purpose. The habit may be beneficial for the trans- 

 mission as in viruses, but this cannot be supposed to be the only reason 

 for the persistence. If eggs of a parasite such as a tapeworm or a flute 

 acquired the faculty to develop in the host bodies without trouble- 

 some host change, it would be said that a remarkable progress was 

 established for their proliferation, but the host should be perished 

 because of the dreadful proliferation of the parasite, resulting in the 

 extinction of the parasite itself. During the long course of evolution, 

 parasite capable of developing without host change might appear, but it 

 seems no doubt that the host of such dreadful parasites would soon 



