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. 
(2) 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 
1838 
: (c) ne 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 
