28 INVERTEBRATE ZOOLOGY 



indeed. On the other hand, those species which undergo con- 

 jugation possess all of the possibilities for recombinations of 

 nuclear material from two parents which characterize the highly 

 complicated mechanism of biparental inheritance in the Metazoa. 

 Intermediate between these two extremes lie those species in 

 which endomixis occurs. This process, with its striking simi- 

 larity if not full equivalence to parthenogenesis in the Metazoa, 

 introduces the possiblility of recombinations of the gene-bearing 

 chromatin within the body of the single individual. 



Many types of bodily change have been recorded among Pro- 

 tozoa, seemingly due wholly to environmental changes such as 

 those in the medium, in temperature, etc. Even certain types 

 of mutilation are passed on to the progeny for a limited number 

 of generations. But the evidence is fairly conclusive that here, 

 as in the Metazoa, changes originating during the life of the indi- 

 vidual are not permanent. When abnormal individuals produced 

 by a changed environment or by mutilation are returned to 

 normal surroundings the body eventually assumes its character- 

 istic form. 



Extensive work on selection has been carried on by H. S. 

 Jennings and others who have found that a single individual of 

 a "wild" population may contain a great number of hereditary 

 characteristics stored up in the single individual through its past 

 history of chromatin interchange during conjugation. When 

 such wild individuals are isolated and their offspring are reared 

 under uniform conditions, extremely diverse individuals may be 

 secured, evidently the result of recombinations of characters. 

 For one species of Paramecium, Jennings discovered eight dis- 

 tinct races each reproducing its own kind. 



Colony formation frequently results from incomplete separa- 

 tion of cells following division. The products of rapid division 

 may remain in union for a short time to form a temporary aggre- 

 gate which ultimately separates into its individual cells, or they 

 may remain permanently associated to form a colony. The con- 

 nections and relationships between the individuals of a colony 

 are highly varied. The cells of the colony, regardless of their 

 arrangement, usually have connections of protoplasm passing 

 from one to another. By this means communication between 

 the cells is carried on readily and the actions of the indi- 

 vidual cells are coordinated so that the group behaves as a 

 unit. 



