300 PROTOZOA 



sufficient genetic variability for adaptation to widely varied con- 

 ditions. They are thus comparable to the sexual outbreeders. On 

 the other hand, asexual diploids are comparable to sexual in- 

 breeders. They cannot achieve so readily genetic adaptation to 

 varied conditions. They are thus confined largely to relatively 

 constant or very slowly changing conditions, such as life in a 

 host or in a considerable body of water which acts as a buf- 

 fer against rapid change. A different sort of genetic basis for 

 variability is found in them: the possession of multiple gene loci 

 for alternative and mutually exclusive traits which are readily 

 transformed one into another in response to different environ- 

 mental conditions. Various intermediate types of Protozoa are 

 found to manifest various combinations of the mechanisms and 

 features just mentioned, e.g., haploid asexuals with small popula- 

 tions and slow reproduction share some of the diploid asexual 

 features, as does the well-developed asexual phase of some sexual 

 Ciliates. 



All systems of breeding and both methods of reproduction have 

 proved themselves by existing and persisting in nature. Each 

 must be well adapted to the conditions under which it is found. 

 What one system or method lacks must be compensated by some 

 other feature it possesses. This does not mean that all systems 

 are equally likely to succeed on the evolutionary time scale. 

 Radical changes in conditions may periodically purge inbreeders 

 and asexuals which flourish under relatively constant conditions. 

 Their specialization to such conditions may give them a short- 

 term advantage over outbreeders. But those inbreeders and 

 asexuals that select the constant environments of hosts have 

 also persisted long on the evolutionary time scale, as is clearly 

 shown by the Flagellate symbionts of roaches and termites. 



The survey of the Protozoa indicates that there an 1 relatively 

 lew fundamental alternatives open to such organisms, perhaps 

 to an\ organisms. On the one hand, they can evolve systems of 

 genetic variability that permit adaptation to a wide range ol 

 conditions of life, as is done by sexual outbreeders and asexual 

 haploids that rapidly produce large populations. On the other 

 hand, the} can evolve highly specialized adaptations to relatively 



