Mat 3, 1907] 



SCIENCE 



697 



eseence and old age, the whole constituting 

 a life history. 



In the protozoan species, according to the 

 old custom in taxonomy, these criteria of 

 species can not be maintained. The indi- 

 vidual, in the first place, has no life history 

 to speak of; it is formed by division of a 

 cell; it lives only a few hours as an inde- 

 pendent cell and then uiisappears, its sub- 

 stance going into two or many new cells. 

 There is no indication of the changes in 

 vitality characteristic of individuals of 

 metazoa, that is, no trace of periods of 

 youth, adolescence and old age while nat- 

 ural death of the individual is unknown. 

 Furthermore, in such individuals the cri- 

 terion of offspring which exactly resemble 

 the parents is not observed, for ceUs may be 

 formed in the course of the several divi- 

 sions which do not at all resemble the 

 parent cell and which seen independently 

 would warrant interpretation as a new 

 species. Thus in Tetramitus or Cerco- 

 monas the perfectly rigid contour of the 

 ordinary 'individual' is very different from 

 the amoeboid 'individual' which is ulti- 

 mately formed. On the basis of such vari- 

 able 'individuals' the protozoan species 

 must be of questionable taxonomic value. 



A new basis for the conception of pro- 

 tozoan species was given by Schaudinn in 

 1900. Always interested more particularly 

 in the life history of protozoa, he gave from 

 time to time more or less complete accounts 

 of the life cycle of different forms, e. g., 

 Calcituba, Leydenia, Polystomella, Para- 

 mceha, Trichosphoerium, etc., while in this 

 year he founded Coccidium schuhergi on 

 the basis of the complete life cycle and gave 

 us a model which later students of the 

 group have tried to follow. 



The new method of taxonomic research 

 which Schaudinn started has resulted in a 

 far more profound knowledge of protozoan 

 species. A number of supposedly different 



varieties, species and even genera have been 

 found to be only stages in some life cycle. 

 For example, microsphsric and macro- 

 sphferic shells of foraminifera are now 

 known to be only stages in the life history 

 of the individual foraminif eron, and in my 

 own experience some of the commonest 

 forms of microscopic life are found to be 

 curiously related. Thus an organism which 

 formerly any student of the protozoa would 

 have described as a species of the genus 

 Pelomyxa, is found to be only a stage in 

 the life history of Amoeba; and the sup- 

 posedly different species of Paramecmm — 

 caudatum and aurelia — are found by con- 

 tinuous culture in their natural habitat to 

 be one and the same species. 



Other examples might be given to show 

 how the 'individual' in the old sense varies 

 from time to time and thus becomes a most 

 unstable subject of protozoan species. For 

 this reason, I have urged that the old idea 

 of the protozoan 'individual' be discarded 

 and the life cycle substituted, and I would 

 recognize as the individual, not the single 

 ceU but the entire aggregate of cells that 

 are formed from the time of one conjuga- 

 tion up to natural death of the protoplasm 

 resulting from it, or until syngamous union 

 of that protoplasm with similar substance 

 from another individual. 



In such an individual we recognize pe- 

 riods of varying vitality which have the 

 same sequence as in metazoa, and we can 

 find characteristic features which indicate 

 the period of youth, of adolescence or old 

 age as in a metazoon. The variations in 

 vitality may be represented by a more or 

 less regular curve in which these periods 

 are clearly marked out. 



It is particularly important that species 

 limits should be clearly defined among the 

 pathogenic protozoa. Here as yet, however, 

 there are very few that are based upon the 

 full life cycle, most of them indeed are on 



