218 PROTOZOA 



factor. Transportation by animal carriers, such as insects, am- 

 phibians, and terrestrial vertebrates, needs to be looked into far 

 more than in the past. Perhaps even occasional hurricanes and 

 floods and subterranean water connections between surface wa- 

 ters play a part. The pressing problem with respect to migration 

 is not just how or how far, but how often. 



The next problem is what happens when a migrant succeeds 

 in getting away from home base into another body of water. This 

 is of course a complex of problems. What happens will depend 

 upon whether or not the new locale contains a population of the 

 same variety. If it does and the migrant is an outbreeder, the 

 events that follow will differ from those that would take place 

 with an inbreeder. Special problems arise if the intruder finds a 

 local population, not of the same variety, but of a variety with 

 which it nevertheless can mate. 



It is clear that such interbreeding varieties can coexist for some 

 time. For example, I have found varieties 4 and 8 in a small sam- 

 ple of water from a single source. Since in the laboratory the hy- 

 brids are readily produced but either die or are very weak, one 

 would expect equilibrium to be reached only when one variety, 

 the less numerous one, dies out completely as a result of such 

 crosses. Persistent coexistence would seem possible only under 

 one of two conditions. (1) If corresponding mating types of the 

 two varieties were in excess, they could not interbreed, and the 

 minority types would be eliminated or kept down by crossing. 

 This would also enforce inbreeding by autogamy in each variety 

 because only one mating type would be present. (2) If there is 

 nonrandom mating between the two varieties and each preferen- 

 tially mates with its own variety, this would reduce the elimina- 

 tion which results from crossing. 



No great problems are posed by the migration of an outbreeder 

 into waters occupied by another strain of the same variety. To the 

 extent that crossing occurs, a new equilibrium of the now com- 

 mon gene pools would occur. The low or nonexistent mortality 

 From strain crosses of outbrecders would impose little or no bar 

 to amalgamation. The situation lor an inbreeding intruder is of 

 course very different. Karyotypic diversity and F2 mortality 



