282 PROTOZOA 



Flagellates (see review by Wenrich, 1954). Whether the second 

 point is to be ascribed to incompleteness of investigation or to 

 actual absence of sexuality in the lower animal Flagellates re- 

 mains unknown. However, many examples of perfectly standard 

 meiotic and fertilization processes are well known among the 

 plant Flagellates. One could hardly assume therefore that 

 sexuality in Flagellates first arose in the complex symbionts of 

 wood roaches. Rather one needs to inquire why so many of the 

 simpler Flagellates lost it, if indeed they do lack it. That the 

 Flagellates of wood roaches show stages in the loss, not the 

 origin, of meiosis and fertilization is further indicated by a com- 

 parison of the situation in wood roaches and termites. 



The wood roaches are primitive members of the group that 

 gave rise to termites. Hence, the later evolutionary stages of the 

 symbionts might be expected in the termites. Closely related 

 Flagellates are indeed also found as symbionts in the termites. 

 But without exception these show no sexual reproduction what- 

 ever. They reproduce asexually only. It is true that Cleveland's 

 evidence correlates the sexual cycles in the Flagellates of wood 

 roaches with the moulting hormones of the roach, and exactly the 

 same hormonal conditions would not be expected in termites. 

 But any organism which is under selective pressure to maintain 

 the hard won evolution of sexuality should not find such a change 

 of hormone pattern beyond its capacity to cope with. More 

 likely, the change in the mode of life of the evolving host pro- 

 vided no more pressure for genetic variability on the symbionts, 

 and possibly less. 



Each colony of termites is isolated underground. The extent to 

 which the brief nuptial flight preceding the founding of new 

 colonies permits outbreeding of the host is not, so far as I am 

 aware, known. It seems that there is a considerable degree of 

 synchronization in swarming, however, so that interbreeding 

 between neighboring colonics might well occur. On the other 

 hand, the enormous number of eggs laid by a single female is a 

 powerful factor for assuring a high degree of uniformity among 

 the members of a colony and its descendant colonies. The rela- 

 tive!) small degree of host variability and presumably very slow 



