Vol. XI. October, 1906. No. 5 



BIOLOGICAL BULLETIN 



THE PROTOZOAN LIFE CYCLE. 



GARY N. CALKINS. 



Twenty years ago it was an amusing pastime to see with the 

 aid of the microscope, and to describe, new and interesting forms 

 of unicellular organisms. Today there is not a field of biological 

 science that is not illumined by the deeper study of the protozoa, 

 and the pastime of our fathers has become the science of protozo- 

 ology to-day. In its modern aspect this new science has many 

 sides, morphology, physiology, psychology, cytology, and pa- 

 thology, and although there is little danger of its being cut up into 

 unrecognizable parts, there is need of some ground principle or 

 principles to hold the many branches of protozoa study together 

 and to unify the science. It was the genius of Schaudinn, whose 

 untimely death has taken from protozoology its most brilliant 

 light, to establish such an unifying foundation, and in his clear 

 perception of the importance of the life cycle we have the key 

 note of our present day conception of the protozoa. With our 

 present knowledge we may define protozoa as : Independent, uni- 

 cellular, animal organisms which reproduce by division or spore 

 formation, the progeny passing through various phases of activity 

 collectively known as the life cycle, and manifesting various 

 degrees of vitality with accompanying form changes. 



In thus emphasizing the life cycle in the definition I would seek 

 to introduce into protozoa study the recognition of the entire cycle 

 of changes as a necessary basis for species. I would have the 

 presentation of the life history of a protozoon a prerequisite for 

 its acceptance as a new species and would have zoologists recog- 



1 Substance of addresses given before Sections D and K of the British Association 

 for the Advancement of Science, at York, 1906. 



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