B 



FIG. 3. A, Myxomycete (a Slime Fungus) ; 

 B, Amoeba (an animal), x 350. 



INTRODUCTORY 3 



find their home in tan-pits and on decaying timber, 

 &c., are, under some conditions and at certain stages 

 in their life-history, quite similar in appearance to 

 some of the simplest animals, so much so, indeed, that 

 many zoologists claim them as members of the animal 

 kingdom (Fig. 3). It has even been suggested that 

 all such lowly 

 organised 

 forms of life, 

 about whose 

 relationship 

 to undoubted 

 plants and 

 animals dif- 

 ference of 

 opinion exists, 

 should be grouped into a sort of " no man's land," 

 a territory inhabited by living things which do not, 

 so to speak, exhibit any pronounced affinities to 

 'either of the adjoining nations. Without entering 

 into a discussion of such dubious cases we may at 

 present simply recognise the existence of the two 

 great groups known as plants and animals respec- 

 tively, each containing organisms of lower and of 

 higher grade, and of gradually increasing complexity 

 of structure, the two series diverging from a neutral 

 region inhabited by forms that fail to show the 

 distinguishing characters either of the one group or 

 of the other. 



Without at present making any attempt to 

 enumerate the marks which distinguish plants, 

 animals and neutrals, let us endeavour to find some 

 character which all of them agree in possessing, vitality. 

 Obviously, they are all alive ; they all possess 

 vitality. But what is meant by vitality ? The 



