XI. EVOLUTION AND MUTATION 405 



raised in the same cultures. He has suggested that the characteristics 

 may be transmissible through a virus-like agent. Again, L'Heritier 

 (78) has published a full account of the behaviour of the "viroid" or 

 cytoplasmic particle responsible for CO. sensibility in Drosophila. 

 The agent is capable of transmitting the sensibility from an sensitive 

 individual to other normals as do usual viruses. 



As already stated, besides virus-like agents hormones are involved 

 in the communication of characters among different cells or organs in 

 a multicellular organsism. Similarly, the balance of castes in a termite 

 society is believed by some workers to be maintained by means of 

 certain special hormones (79). 



The castes of termites develop according to the needs of the colony, 

 just as the embryonic cells of a single animal differentiate into blood 

 cells, bone cells, muscle cells and so on. Thus a colony of a termite 

 is a kind of superorganism. The caste of a termite is not fixed ; any 

 nymph in a colony of termites can develop into a soldier or a king or 

 queen of the supplementary type, developing on what the situation 

 demanded. Such a development is controlled by means of some kind 

 of the so-called social hormone, produced by the differentiated adults 

 and acted upon the undifferentiated nymphs. When the colonies were 

 separated by a screen which prevented any contact, the orphanized 

 colony was produced and supplemented its own king and queen in the 

 normal way. An active principle transmitted by contact, that is, a 

 social hormone, is responsible for the development of differentiation. 



As fully discussed in Chapter II, at an early stage of the develop- 

 ment embryonic cells are endowed with unstable structures and so can 

 change in every direction according to the change of environmental 

 factors, but as the development advances, the structures will become 

 gradually fixed and unchangeable. Entirely the same phenomenon is 

 recognized in the development of the castes of a termite society. Thus, 

 probability of change in a nymph decreases with time after molt. With 

 the proper stimuli all nymphs that have just molted will change, but 

 of the nymphs that molted 19 days earlier, only 50 per cent will change 

 and 50 days earlier less than 20 per cent. 



These facts clearly show that the castes of termites correspond to 

 cells or organs in an ordinary multicellular organism, which the latter 

 in turn is equivalent to a colony, and that the development of the 

 castes is achieved by the same principle as that of the cells of an 

 embryo. Therefore, the above concept that the transmission of the 

 characters of soldiers and workers to the king and queen is achieved 

 by "virus" seems to be only natural. 



It is well known that numerous varieties of flowering plants are 

 attributed to virus infection. The most usual effect of a virus infection 



