GREAT STEPS IN EVOLUTION 77 



ment. Lastly, the living creature has a 

 persistent unified behaviour, a power of 

 profiting by experience, a creative capacity 

 as a genuine agent. 



ORIGINS AMONG THE PROTOZOA. It is well 

 over two centuries since the Dutch microscop- 

 ist, Leeuwenhoek, exhibited to the Royal 

 Society of London some of those unicellular 

 animals or animalcules which we now call 

 Protozoa the Fellows present signing an 

 affidavit that they had really seen the minute 

 creatures. This was the beginning of a study 

 which has been extraordinarily fertile in itself 

 and in its bearing on other lines of research. 

 As has become so emphatic recently, the study 

 is one of enormous practical importance to 

 man, since some of the most terrible diseases, 

 such as malaria and sleeping sickness, are 

 due to Protozoa, but the study is also of 

 fundamental theoretical importance. For the 

 Protozoa give us, so to speak, a natural 

 analysis of the elements which compose the 

 higher animals; the phases of their life-cycles 

 are sometimes echoed in the cellular variations 

 of man himself; a few of them seem to linger 

 in a state of relative simplicity, approximat- 

 ing to that which must have characterized 

 the true Protozoa, or first animals; they are, 

 as it were, permanent germ-cells which never 



