No. 593] THE PEOTOZOAX LITE CYCLE 



low fever, malaria and dysentery; lias rehabilitated vast 

 tracts of land in Italy ; saved millions of dollars in South 

 Africa and in our southern states, and has made the 

 Panama Canal possible. 



Such are the first, and practically the most important, 

 results of our knowledge concerning protozoan life cycles ; 

 quite enough, indeed, to justify the science of Protozo- 

 ology. Important as these results are, we are not at all 

 satisfied ; we know too little about the conditions of devel- 

 opment ; too little about the nature of the vital processes 

 of the organisms themselves and their variations in struc- 

 ture and function under differing conditions, ignorance 

 which must be cleared away before much further practical 

 advance can be made. Further advance will be less spec- 

 tacular and must be based upon the biological study of 

 the organisms as units of protoplasmic substance, and 

 this will rest upon working hypotheses supported by ex- 

 periment. It is along such theoretical lines that I wish to 

 direct your attention for a few minutes, to develop a con- 

 ception of the life cycle as a whole, and to offer a theo- 

 retical interpretation of the different phases of vitality 

 and of structural variations. 



Let us consider for a moment, a single Ameba or a 

 malaria germ, not as a cause of disease, but as a unit mass 

 of protoplasm which, like a free-living Paramecium or 

 Didinium, performs all of the fundamental vital activities 

 common to living things, namely nutrition, excretion, irri- 

 tability and reproduction. The chemical composition of 

 these unit masses, so far as I know, has never been made 

 out, but there is no reason to doubt that it agrees with 

 that of other living substances, since the accompanying 

 properties of protoplasm— metabolism, growth and re- 

 production—are obviously performed, and probably in 

 the same way. In such unit masses of protoplasm we 

 assume that processes of hydrolysis, synthesis, oxidation 

 and reduction, are constantly going on as in other proto- 

 plasms, and not in any haphazard way, but always orderly 

 and under regulative control of the organism as a whole. 



