232 GARY N. CALKINS. 



of a clear conception of the different phases of the parasite, and 

 the possibility of different hosts or of different effects on the same 

 hosts at different periods of vitality should be known to the 

 pathologists. The tendency to put protozoa on the same basis 

 of research as the bacteria, despite the brilliant work in cultivat- 

 ing certain types of protozoa on artificial media, which one of my 

 countrymen regards as the sine qua non of pathogenic protozoan 

 research, seems to me to be a step in the wrong direction. To 

 study parasitic protozoa in culture is to study them under one 

 condition only, and in one phase only of the life history, and the 

 different forms that are met with in such artificial media may be 

 more often involution types than normal phases, and the great 

 multiplication of species of Trypanosoma or Spirocliceta bespeaks 

 perhaps more than any other one thing the presence of a new 

 type of novitiate in protozoan research. 



If the life cycle is to be accepted as the basis of new species, 

 and regarded as the individual in a taxonomic sense, it should 

 be sufficiently definite to be taken as a unit, and the life his- 

 tories of widely separated species should have some common 

 grounds for comparison. Thanks to the great stimulus given 

 in recent years to protozoan study, we know the full life his- 

 tory of many widely separated forms, and at the present time 

 we are able to generalize to some extent and to formulate a few 

 ground principles. The old-time comparison of the metazoon 

 with the mass of cells that are formed by the repeated division 

 of the first parent cell of a protozoon after conjugation, can be 

 amplified and extended at the present time to the comparison 

 with the metazoon of not only the mass or morphology, but the 

 physiology or general biology of the constituent cells as well. 

 As with the metazoon so with the aggregate of protozoa cells, 

 we note a period of youth characterized by active cell -prolifera- 

 tion ; this in both groups of organisms is followed by the gradual 

 loss of the division energy accompanied by morphological changes 

 in type of the cells preliminary to conjugation and fertilization 

 and to the renewal of vitality by this means. When such re- 

 newal is omitted, and for one reason or another this stage is 

 never reached by the great majority of protozoa, the third char- 

 acteristic period old age supervenes and the race of pro- 



