276 IV. THE PRINCIPLES OF LIFE PHENOMENA 



generation through the eggs. The character to produce the micro- 

 organisms, therefore, must be the inheritable character essential for 

 the insects. This fact strongly suggests that certain fungi and 

 bacteria are still now succeeding to the habitude of viruses, their 

 ancestors, to be rejuvenated. 



Leach (85) has already expressed, the opinion that insects are not 

 merely disseminators of inoculum in the case of pathogenic fungi, but 

 that the insect-fungi relationship is highly organized and has broad 

 biological and evolutionary significance. The fact that over 200 kinds 

 of bacteria have been isolated from insects may show what a great 

 number of bacteria are making use of insects for their rejuvenation. 

 As we shall see later, spirochaetes are also regarded as microorganisms 

 succeeding to this habitude ; meanwhile about 20 kinds of them have 

 been found in insects. 



Many examples are known, as pointed out already, to show. that 

 insects are not afflicted by viruses harbouring in them, in some cases 

 the relationship between them being regarded even as a symbiosis. 

 Such a relationship appears to be kept on even after the viruses have 

 evolved into undoubted microorganisms. It cannot be said, however, 

 that insects are always not afflicted by microorganisms. Numerous 

 infectious diseases due to bacteria or fungi are well known. 



2. The Rejuvenescence of Tubercle Bacillus 



As already dealt with, a phenomenon related to sexual conjugation 

 is occasionally observed with colon bacillus, but it cannot be considered 

 that the bacillus is provided with the distinct habitude to be rejuve- 

 nated regularly by the sexual process. The observed phenomenon 

 should be looked upon as only an extremely primitive form of the 

 sexual conjugation, and presumably the bacillus is usually not re- 

 juvenated by such a process. 



Here again, it seems probable that this bacillus is also resorting 

 to the means of rejuvenation which their ancestors used habitually. 

 It has been reported that the normal human individual harbours two 

 types of colon bacillus, residents and transients, i. e., persistent over 

 long periods strain and limited to a few days strain, the former con- 

 tains typically one or two strains (86). This pattern of bowell bacilli 

 is shown by very young babies as well as adults. Babies as intimately 

 associated as twins harbour the same resident. 



Colon bacillus, however, may not be so adequate as tubercle 

 bacillus to be dealt with concerning this subject. Almost all the 

 human beings are infected with tubercle bacillus as with measles 



