192 THE STUDY OF ANIMAL LIFE CHAP. 



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 in- 

 come 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 con- 

 servative and miserly than the liberally spendthrift 

 animals, and it is possible that some of the most charac- 

 teristic possessions of plants, e.g. cellulose, may be 

 chemical expressions of a marked preponderance of con- 

 structive and up-building vital processes. 



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, such as a water- 

 flea ; how does it differ from one of the simplest animals, 

 or Protozoa ? It consists of manv units of living matter 



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instead of only one. The Protozoa are single cells, all 

 the others from sponge to man are many-celled. The 

 Protozoa are units ; all others the Metazoa are com- 

 posite aggregates of units, or cities of cells. But since 

 the Protozoa are complete organisms, often intricate in 

 structure and behaviour, it may be more accurate to 

 call them non-cellular. 



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, draw- 

 ing 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 multiply. But in the single-celled Protozoon 

 all the processes of life occur within a unit mass of living 

 matter. In the many-celled Metazoon the various pro- 



