PLANTS AND ANIMALS. 23 



the squirrel on its branches. In regard to none of the main 

 functions (except excretion) is there any essential difference. 

 Many simple plants swim about actively ; young shoots and 

 roots also move ; and there are many cases in which even 

 the full-grown parts of plants exhibit movement. Moreover, 

 the tendrils of climbers, the leaves of the sensitive plant, the 

 tentacles of the sundew, the stamens of the rock-rose, the 

 stigma of the musk, and many other plant structures exhibit 

 marked sensitiveness. 



(b) Resemblance in structure. The simplest plants (Pro- 

 tophyta), like the simplest animals (Protozoa), are single cells ; 

 the higher plants (Metaphyta) and higher animals (Metazoa) 

 are built up of cells and various modifications of cells. In 

 short, all organisms have a cellular structure. This general 

 conclusion is part of the Cell Theory or Cell Doctrine 



(c) Resemblance in development. When we trace the 

 beech-tree back to the beginning of its life, we find that it 

 arises from a unit element or egg-cell, which is fertilised by 

 intimate union with a male element derived from the pollen- 

 grain.. When we trace the squirrel -back to the beginning 

 of its life, we find that it also arises from a unit element or 

 egg-cell, which is fertilised by intimate union with a male 

 cell or spermatozoon. Thus all the many-celled plants and 

 animals begin as fertilised egg-cells, except in cases of 

 virgin birth (parthenogenesis) or of asexual reproduction. 

 From the egg-cell, which divides and redivides after fertilisa- 

 tion, the body of the plant or animal is built up by con- 

 tinued division, arrangement, and modification of cells. 



Contrasts. But while there is no absolute distinction 

 between plants and animals, they represent divergent 

 branches of a V-shaped tree of life. It is easy to distinguish 

 extremes like bird and daisy, less easy to contrast sponge 

 and mushroom, well-nigh impossible to decide whether 

 some very simple forms, which Haeckel called " Protists," 

 have a bias towards plants or towards animals. We cannot 

 do more than state average distinctions. The food which 

 most plants absorb is cruder or chemically simpler than that 

 which animals are able to utilise. Thus most plants derive 

 the carbon they require from the carbon dioxide of the air, 

 while only a few (green) animals have this power ; all the 



