IX. EVOLUTION AND MUTATION 409 



of assimilase, or until the assimilases were endowed with characters 

 as primitive organisms. The progress of the evolution in such a man- 

 ner, however, was most probably highly tedious, and hence a dreadfully 

 long span of time might pass between the appearance of assimilase-like 

 protein aggregates and the production of the organism-like assimilases. 

 The very remarkable evolution mechanism belonging to the present 

 organisms was presumably advanced thereafter, and since the mecha- 

 nism might be advanced the more, the more evolved the organisms, the 

 rate of the evolution of the modern organisms may be extremely great 

 as compared with that of primeval organisms. 



It seems reasonable to regard the acquisition of resistance of bacteria 

 to several antibacterial drugs as a type of the adaptation seen in higher 

 organism, but a number of authors claim that the resistance is at- 

 tributed to spontaneous mutation and natural selection, that is, the 

 emergence of a population resistant to a given drug results from the 

 selective effect of that agent upon mutants arising spontaneously from 

 the original population (82). Since in primitive organisms such as 

 bacteria a highly advanced mechanism of the evolution cannot be ex- 

 pected, adaptation may be not so distinct as in higher organisms. But 

 it should be borne in mind that adaptability is one of the basic char- 

 acters of protein itself and so the protein molecule itself can be adapted 

 to environments though in an undeveloped way. 



Mutation may be looked upon, in a sense, as a generation of a 

 virus as discussed already. Hence, the primary organisms in extremely 

 primitive stages may be said to have been evolved by the successive 

 production of viruses. Accordingly the primary organisms may be consi- 

 dered as being originated from viruses like the secondary. In this 

 respect, there appears no fundamental difference between the primary 

 and the secondary organisms. 



In fact, it may be difficult to draw a distinct line of demarkation 

 between these two main groups of organisms. It may be said, however, 

 that the secondary organisms were generated on the basis of preexisted 

 protoplasm or organisms, whilst the basis on which the primary org- 

 anisms were de^'^eloped was the incomplete protoplasm or the aggrega- 

 tion of protein molecules which had been produced without organisms. 

 Therefore, the secondary organisms unlike the primary have their host 

 at least when they are in their primitive stages. Nevertheless, in the 

 case of the secondary organisms that developed on the basis of the 

 primary organisms existing at the most primitive stage, there might 

 be no distinct difference between the parasite and the host, and so the 

 parasitic nature would be insignificant in such secondary organisms. 



In this respect it may be suggested that the Alollusca may be the 

 oldest secondary organisms, because their parasitic nature seems only 



