76 EVOLUTION 



haviour, 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 micro- 

 scopist, 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 mi- 

 nute 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 re- 

 cently, the study is one of enormous practi- 

 cal 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 him- 

 self; a few of them seem to linger in a state 

 of relative simplicity, approximating to that 

 which must have characterized the true 

 Protozoa, or first animals; they are, as it 

 were, permanent germ-cells which never 

 get beyond the ovum and sperm stage; and 

 they show us the beginnings of division of 



