THE PROTOZOA-THE DAWN OF LIFE 29 



only with the sedentary stalked form ; so that in Vorticella we 

 have the sexes distinctly differentiated in form and habit prior 

 to conjugation. 



In the foregoing outline sketch of the Protozoa, attention has 

 been drawn, so far as space and the avoidance of very technical 

 questions would permit, to the most interesting points concern- 

 ing these remarkable organisms. In the Amoeba we have had an 

 example of a very primitive form of life, of an organism complete 

 in itself, though of comparatively simple structure, capable of 

 carrying on all the processes of life the capture and assimilation 

 of food, movement, growth, multiplication without the develop- 

 ment of any special organs for carrying out these vital functions, 

 excepting, of course, the part which the nucleus plays in its mul- 

 tiplication by binary fission. And we have seen the comparative 

 immortality of these organisms. In the fusing or conjugation of 

 similar cells, identical in appearance, we may trace the dawn of 

 sex and the gradual differentiation into the typical active male 

 and sedentary female cells (sperm and ovum) ; while in the colonial 

 form of Volvox we caught a glimpse of the introduction of death 

 as a constant phenomenon, through the modification to special 

 ends of certain cells. Of the economic importance of the study 

 of certain groups of the Protozoa, the malarial parasite and the 

 Trypanosome of sleeping sickness are sufficiently impressive 

 examples. Indeed, the story of the discovery of the malarial 

 parasite, of its sexual cycle within the gut of the spot-winged mos- 

 quito, and the means that discovery has placed in our hands of 

 successfully combating one of the most dreaded tropical diseases, 

 and of thereby bringing health and prosperity to wide regions 

 of country, should eloquently convince even the most hardened 

 sceptic of the inestimable value of biological research. 



