CHAP. XI The Elements of Structure iTi 



The net result of this contrast is that animals are more 

 active than plants. Life slumbers in the plant ; it wakes 

 and works in the animal. The changes associated with the 

 living matter of an animal are seemingly more intense and 

 rapid ; the ratio of disruptive, power-expending changes to 

 constructive power-accumulating changes is greater ; most 

 animals live more nearly up to their income than most 

 plants do. They live on richer food ; they take the pounds 

 which plants have accumulated in pence, and spend them. 

 Of course plants also expend energy, but for the most part 

 within their own bodies ; they neither toil nor spin. They 

 stoop to conquer the elements of the inorganic world, but 

 have comparatively little power of moving or feeling. They 

 are more conservative and miserly than the liberally spend- 

 thrift animals, and it is possible that some of the most 

 characteristic possessions of plants, e.£: cellulose, may be 

 chemical expressions of a marked preponderance of con- 

 structive and up-building vital processes. It is enough, 

 however, if we have to some extent realised the common- 

 places that plants and animals live the same sort of life, 

 but that the animals are on an average more active and 

 wide-awake than the plants. 



2. The Relation of the Simplest Animals to those 

 which are more Complex. — From the pond-water catch in 

 a glass tube one of the small animals, suppose it be a tiny 

 water-flea or a minute " worm " ; how does it differ from one 

 of the simplest animals, such as an Infusorian ? It consists of 

 many units of living matter instead of only one. The con- 

 trast is like that between an egg and the bird which is 

 hatched from within it. The simplest animals are single 

 cells, all the others from sponge to man are many-celled. 

 The Protozoa are units ; all others — the Metazoa — are 

 composite aggregates of units, or cities of cells. 



Compare the life of one of the Protozoa with that of 

 a worm, a frog, or a bird. Both are alive, both may be 

 seen moving, shrinking away from what is hurtful, drawing 

 near. to what is useful, engulfing food, and getting rid of 

 refuse. Both are breathing, for carbonic acid will poison 

 them, and dearth of oxygen will kill them ; both grow and 



