X. THE SECONDARY ORGANISMS 205 



possible to suppose that all the plants may be the secondary. The 

 thallophyta, to which algae and fungi belong, include plants of great 

 diversity, but many of which have little in common save the small 

 size and simplicity. 



On the other hand, it is a noteworthy fact that there is entirely 

 no parasitic species in chordates, the most advanced animals, among 

 the classes of which the most orderly phylogenetic relation can be 

 found. This suggests that chordates are the primary organisms which 

 might provide the first scaffolding to the secondary organisms for 

 their evolution as well as for their generation. Moreover, it should 

 be noted that no species of parasitic echinoderms is known, although 

 certain kinds of echinoderms provide the host to parasitic snail, but 

 they themselves are never parasitic, whereas it is generally accepted 

 that there seems a closely related phylogenetical connection between 

 echinoderms and chordates. This fact also suggsts that echinoderms 

 as well as chordates are the primary organisms. 



Now we must consider in this connection of mollusks, one of the 

 most highly organized phyla of animals. It is remarkable that differ- 

 ent groups of mollusks are parasitic. In unionidae parasitism is re- 

 sorted to by larval forms only, known as glochidia, parasitic on fishes, 

 Avhereas in parasitic snails the adults are parasitic and the larval 

 forms free-living, the hosts being always provided by echinoderms 

 (117). In contrast to arthropods, however ; the parasitic nature of 

 mollusks is rather exceptional, only being observed with some 

 peculiar kinds. Nevertheless, since parasitism does exist in mollusks, 

 if as relics, it may be not unreasonable to regard mollusks as the 

 biggest senior of the secondary animals that were evolved from 

 viruses in the oldest age. 



Of course, if a secondary organism was developed to a certain 

 extent, viruses would naturally be generated in it, and the viruses in 

 turn would be evolved into organisms in this senior secondary orga- 

 nism, and hence it cannot be said that the secondary organisms are 

 always generated and evolved in the primary organisms. 



The writer thus has come to possess the strong view that viruses 

 represent a transitional stage from non-living to living whereas many 

 authors consider that viruses have evolved by a process of retrograde 

 evolution from higher organisms, that is, present-day viruses are 

 direct descendants of peculiar forms that were once free-living. This 

 idea appears to be based upon the fact that saprophytic organisms 

 sometimes lose the ability to synthesize certain essential growth re- 

 quirements, involving the loss of structure and functions that are no 

 longer needed, thus being degenerated both in their function and forms 

 to become apparently very primitive organisms. They believe that in 



