212 LECTUEES ON BIOLOGY 



existence. But as the children are never an exact replica of the 

 parents but differ from them in one or another characteristic, 

 there must already have been inherent in the ' primordial slime ' 

 the possibility of variation. If that were not the case evolution 

 of the organic world could not have taken place, and it would 

 bear to-day the same primitive appearance as in the ages in which 

 it came into existence. 



Most zoological text-books divide the animal kingdom into two 

 great divisions. On one side are the higher multicellular animals, 

 among which we find all animal organisms, from the little polyp 

 whose body consists merely of a cavity surrounded by a double 

 cell-layer, up to man with his body complex with numerous 

 tissues and organs. On the other side are the unicellular 

 protozoa. 



The name protozoon always seems to create in the lay mind 

 erroneous ideas. It came into use at a time when our knowledge 

 of the structure of the protozoa was of rather primitive nature, 

 and when serious men still believed that the Amoebae and Infuso- 

 rians owed their existence to spontaneous generation, developing 

 everywhere in decaying substances. To-day we know that the 

 unicellular organisms are no more original forms than are 

 mammals or insects. Like the latter they bear unmistakable 

 signs proving that their species have undergone a development 

 extending over vast periods. But all vital functions go on in 

 the protozoa in a much clearer and simpler manner than in the 

 multicellular animals, and a study of their reproduction throws 

 a clearer light upon the conditions obtaining in the higher 

 animals. It is one of the greatest joys of biological research to 

 penetrate with the aid of the microscope into this rich world of 

 invisible life and to discover its secrets. 



In a drop of water taken from any ditch or pond we are able 

 to observe with the naked eye a minute white spot. This little 

 insignificant thing holds within itself all the wonders of life. 

 All the numerous functions of respiration, nutrition, locomotion, 

 and reproduction which are performed in man by highly complex 

 organ- systems, are going on in this minute drop of transparent 

 slime in a different yet none the less perfect manner. 



The little organism before us is a member of the so-called 

 Khizopods, an Anloeba. It owes its name ' change ' to the fact 



