146 GENERAL ZOOLOGY 



animals there are only a very few cases of parthenogenesis, 

 or development of ova without fertilization, and consider- 

 ing the whole animal kingdom there are very few cases in 

 which new individuals do not originate from zygotes. 

 Another view regarding the effects of conjugation has been 

 advocated by a number of zoologists, i.e., " it is more prob- 

 able that conjugation results in increased powers of adapta- 

 bility than in rejuvenescence." 



If protozoans are always single cells, it is but natural to 

 ask what their psychic capabilities may be. Can they 

 profit by experience? Have they memory? In connection 

 with the discussion of Amoeba and Vorticella it has already 

 been shown that protozoans can vary their behavior some- 

 what to meet changes in surrounding conditions, but for 

 the most part their activities are simple and are repeated 

 over and over if stimulating conditions are repeated. A 

 free-swimming individual will start in a certain direction 

 and keep going until it finds food (or a mate, if the breed- 

 ing instincts are active). If some unfavorable stimulus is 

 encountered, however, it backs up a little and starts off 

 in a new direction. If conditions are again unfavorable, 

 there is more backing and trying, until a proper course is 

 found. The life of an active protozoan, then, is a series 

 of continual testings of the surroundings ; success is attained 

 by holding to what promises to be good and avoiding what 

 appears to be bad. There is no reasoning, no planning 

 or forethought. 



The Protozoa present one other point of general interest 

 they were doubtless the ancestors of the Metazoa. Many 

 protozoans have almost become many-celled animals, in 

 fact, it is hard sometimes to draw the line between single 

 and many-celled forms, but none have quite attained 

 the goal. Most of them have taken the wrong road by 

 specializing the structures of single cells, instead of form- 

 ing a group of cells and dividing the work to be done among 

 them. Perhaps many-celled animals have arisen from more 

 than one group of protozoans. The possible steps by which 

 the Metazoa arose will be considered in the next chapter. 



